[clang] [llvm] InstCombine: Use SimplifyDemandedFPClass on fmul (PR #177490)

[Florian Hahn via llvm-commits]

fhahn wrote:

It looks like this is causing a crash for

```
; opt -p instcombine
define float @foo(i1 %cmp, float %x, float noundef %y, float noundef %z) {
entry:
  %div = fdiv float %y, %y
  %scale.0 = select i1 %cmp, float %div, float %y
  %mul = fmul float %scale.0, %x
  ret float %mul
}
```

Fails with

```
Instruction does not dominate all uses!
  %1 = select i1 %0, float 0x7FF8000000000000, float 1.000000e+00
  %scale.0 = select i1 %cmp, float %1, float %y
LLVM ERROR: Broken module found, compilation aborted!
```

https://github.com/llvm/llvm-project/pull/177490