Remove excessive null? checks

[anton-trunov]

For instance, this snippet

fun foo(s: SomeStruct?): Bool {
    if (s == null) { return false }
    else { return s!!.b }
}

translates into the following FunC code:

int $global_foo(tuple $s) impure inline_ref {
    var ($s) = $s;
    if (null?($s)) {        ;; 1st null? check
        return false;
    } else {
        return $SomeStruct$_get_b($SomeStruct$_not_null($s));
    }
}

((int, int)) $SomeStruct$_not_null(tuple v) inline {
    throw_if(128, null?(v));     ;; 2nd null? check
    var (int vvv'i, int vvv'b) = __tact_tuple_destroy_2(v);
    return (vvv'i, vvv'b);
}

which just wastes gas.

[Gusarich]

Do you mean we should generally remove this throw_if(128, null?(v)) for all !! expressions or only in cases when the value was checked for null already?

[anton-trunov]

I would argue we should remove all those checks now and provide a static analysis to warn about unguarded uses of the !! operator