Chain of read+writes could be improved by analyzing array/map indices

[msooseth]

As documented in:

-- This does not strip writes that cannot possibly match a read, in case there are
-- some write(s) in between that it cannot statically determine to be removable, because
-- it will early-abort. So (load idx1 (store idx1 (store idx1 (store idx0)))) will not strip
-- the idx0 store, in case things in between cannot be stripped. Potential for improvement. TODO.
-- See simplify-storage-map-todo test for an example where this happens
readStorage :: Expr EWord -> Expr Storage -> Maybe (Expr EWord)
readStorage w st = go (simplify w) st

A good example test where this is obvious:

    , test "simplify-storage-map-todo" $ do
       Just c <- solcRuntime "MyContract"
        [i|
        contract MyContract {
          mapping (uint => uint) a;
          mapping (uint => uint) b;
          function fun(uint v, uint i) public {
            require(i < 1000);
            require(v < 1000);
            a[i] = v;
            b[i+1] = v+1;
            b[i+v] = 55; // note: this can overwrite b[i+1], hence assert below can fail
            assert(a[i] == v);
            assert(b[i+1] == v+1);
          }
        }

Which will do things like below, where we read from 0x1, and write to a bunch of 0x1, but ALSO write to 0x0, which could be stripped.

                            (SLoad
                              slot:
                                (Keccak
                                  (WriteWord
                                    idx:
                                      0
                                    val:
                                      (Add
                                        1
                                        (Var "arg2")
                                      )
                                  )
                                  (ConcreteBuf
                                    Length: 64 (0x40) bytes
                                    0000:   00 00 00 00  00 00 00 00  00 00 00 00  00 00 00 00   ................
                                    0010:   00 00 00 00  00 00 00 00  00 00 00 00  00 00 00 00   ................
                                    0020:   00 00 00 00  00 00 00 00  00 00 00 00  00 00 00 00   ................
                                    0030:   00 00 00 00  00 00 00 00  00 00 00 00  00 00 00 01   ................
                                  )
                                )
                              storage:
                                (SStore
                                  slot:
                                    (Keccak
                                      (WriteWord
                                        idx:
                                          0
                                        val:
                                          (Add
                                            (Var "arg2")
                                            (Var "arg1")
                                          )
                                      )
                                      (ConcreteBuf
                                        Length: 64 (0x40) bytes
                                        0000:   00 00 00 00  00 00 00 00  00 00 00 00  00 00 00 00   ................
                                        0010:   00 00 00 00  00 00 00 00  00 00 00 00  00 00 00 00   ................
                                        0020:   00 00 00 00  00 00 00 00  00 00 00 00  00 00 00 00   ................
                                        0030:   00 00 00 00  00 00 00 00  00 00 00 00  00 00 00 01   ................
                                      )
                                    )
                                  val:
                                    55
                                )
                                (SStore
                                  slot:
                                    (Keccak
                                      (WriteWord
                                        idx:
                                          0
                                        val:
                                          (Add
                                            1
                                            (Var "arg2")
                                          )
                                      )
                                      (ConcreteBuf
                                        Length: 64 (0x40) bytes
                                        0000:   00 00 00 00  00 00 00 00  00 00 00 00  00 00 00 00   ................
                                        0010:   00 00 00 00  00 00 00 00  00 00 00 00  00 00 00 00   ................
                                        0020:   00 00 00 00  00 00 00 00  00 00 00 00  00 00 00 00   ................
                                        0030:   00 00 00 00  00 00 00 00  00 00 00 00  00 00 00 01   ................
                                      )
                                    )
                                  val:
                                    (Add
                                      1
                                      (Var "arg1")
                                    )
                                )
                                (SStore
                                  slot:
                                    (Keccak
                                      (WriteWord
                                        idx:
                                          0
                                        val:
                                          (Add
                                            1
                                            (Var "arg2")
                                          )
                                      )
                                      (ConcreteBuf
                                        Length: 64 (0x40) bytes
                                        0000:   00 00 00 00  00 00 00 00  00 00 00 00  00 00 00 00   ................
                                        0010:   00 00 00 00  00 00 00 00  00 00 00 00  00 00 00 00   ................
                                        0020:   00 00 00 00  00 00 00 00  00 00 00 00  00 00 00 00   ................
                                        0030:   00 00 00 00  00 00 00 00  00 00 00 00  00 00 00 01   ................
                                      )
                                    )
                                  val:
                                    (Add
                                      1
                                      (Var "arg1")
                                    )
                                )
                                (SStore
                                  slot:
                                    (Keccak
                                      (WriteWord
                                        idx:
                                          0
                                        val:
                                          (Var "arg2")
                                      )
                                      (ConcreteBuf
                                        Length: 64 (0x40) bytes
                                        0000:   00 00 00 00  00 00 00 00  00 00 00 00  00 00 00 00   ................
                                        0010:   00 00 00 00  00 00 00 00  00 00 00 00  00 00 00 00   ................
                                        0020:   00 00 00 00  00 00 00 00  00 00 00 00  00 00 00 00   ................
                                        0030:   00 00 00 00  00 00 00 00  00 00 00 00  00 00 00 00   ................
                                      )
                                    )
                                  val:
                                    (Var "arg1")
                                )
                                (AbstractStore (SymAddr "entrypoint") Nothing)
                            )