Optimize eqz by general getFallthrough in computing condition

[xuruiyang2002]

Currently precompute doesn't do math computations with side-effect like seeing eqz(x) == 0 (x is 1 but has effect) and eqz(y) == 1 (y is 0 but has effect).

For example, it cannot replace

(local.tee $x
 (i32.const 1)
)

=>

(block
 (local.tee $x
  (i32.const 1)
 )
 (i32.const 1)
)

It only replaces entire expressions.

As discussed in #7492

-O3 --gufa -O3 -O3 has the same result as -O2, as does --flatten -O3. It is unfortunate that so much effort is needed. Yes, this should be improved.

So we let OptimizeInstructions to do more here.

Fixes: #7492

[xuruiyang2002]

I've updated the comments as you suggested 👍.