[llvm] [InstCombine] Do not require nsz for reassociating fmul (PR #93698)

[Jay Foad via llvm-commits]

jayfoad wrote:

Here's an Alive2 script to prove that reassociating multiplication of +/- 0 still gives the right result.
```
define float @src(i1 %ia, i1 %ib, i1 %ic) {
  %fa = sitofp i1 %ia to float
  %fb = sitofp i1 %ib to float
  %fc = sitofp i1 %ic to float

  ; za, zb, zc are all +/-0
  %za = fmul float %fa, 0.0
  %zb = fmul float %fb, 0.0
  %zc = fmul float %fc, 0.0

  ; (za * zb) * zc
  %x = fmul float %za, %zb
  %y = fmul float %x, %zc
  ret float %y
}

define float @tgt(i1 %ia, i1 %ib, i1 %ic) {
  %fa = sitofp i1 %ia to float
  %fb = sitofp i1 %ib to float
  %fc = sitofp i1 %ic to float

  ; za, zb, zc are all +/-0
  %za = fmul float %fa, 0.0
  %zb = fmul float %fb, 0.0
  %zc = fmul float %fc, 0.0

  ; za * (zb * zc)
  %x = fmul float %zb, %zc
  %y = fmul float %za, %x
  ret float %y
}
```
If I [try it online](https://alive2.llvm.org/ce/z/z5CCVV) I currently get "ERROR: SMT Error: invalid extract application". @nunoplopes @regehr is that an Alive2 bug?

https://github.com/llvm/llvm-project/pull/93698