Implement optimizations for i32.eqz translation.

[Robbepop]

When translating a Wasm i32.eqz instruction we can view it as negation of local conditionals that can be in the form of i32.eq, i32.lt_s, i64.ge_u etc. and in some cases also i32.{and, or, xor}. Instead of simply emitting a wasmi IR i32.eq x imm(0) instruction we can sometimes instead negate the previous instruction thus emitting fewer instructions in total.

Although this optimization might be mitigated by Wasm codegens that already perform this optimization. More research is needed here.

Affected Wasm instructions if they come after an i32.eqz or i64.eqz:

• br_if: Compile as br_if with negated conditional.
• select: Compile as select with swapped operands. (equal to negated)

Affected Wasm instruction if they come before an i32.eqz or i64.eqz:

• and, or: Compile as not-and or not-or. (to be further optimized by br_if)
• Any Wasm comparison instruction such as i32.eq, i32.lt_s, i64.ge_u etc..

[Robbepop]

The br_if enabled optimizations are going to be covered by #791.

[Robbepop]

Generally this is just about i32.eqz + select after merging #791 which is a rather uncommon use case and not really important enough to introduce yet another bunch of new fused instructions. So we close this.