Add havoc tests

[DavePearce]

Given that we can now infer the position of havoc statements, this means we can also now test the inference algorithm. Some tests to start with:

.code
        push 0x10
        calldatasize
        push loop
        jumpi
        pop
        push 0x5
loop:
        jumpdest
        havoc 0
        dup1
        iszero
        push exit
        jumpi
        push 0x1
        swap1
        sub
        push loop
        jump
exit:
        stop

.code
        calldatasize
        push 0x0
loop:
        jumpdest
        dup2
        dup2
        eq
        push exit
        jumpi
        push 0x1
        add
        push loop
        jump
exit:
        stop

[DavePearce]

And another:

.code
        calldatasize
loop:
        jumpdest
        dup1
        iszero
        push exit
        jumpi
        push 0x1
        dup2
        sub
        swap1
        pop
        push loop
        jump
exit:
        stop