[llvm] [InstCombine] Ensure Safe Handling of Flags in foldFNegIntoConstant (PR #94148)

[via llvm-commits]

SahilPatidar wrote:

This is a test we are breaking:
```llvm
; With nsz and reassoc: Y - ((X * 5) + Y) --> X * -5

define float @sub_add_neg_x(float %x, float %y) {
; CHECK-LABEL: @sub_add_neg_x(
; CHECK-NEXT:    [[R:%.*]] = fmul float [[X:%.*]], -5.000000e+00
; CHECK-NEXT:    ret float [[R]]
;
  %mul = fmul float %x, 5.000000e+00
  %add = fadd float %mul, %y
  %r = fsub nsz reassoc float %y, %add
  ret float %r
}
```

https://github.com/llvm/llvm-project/pull/94148