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

[Jay Foad via llvm-commits]

jayfoad wrote:

> Since it was breaking the `fsub` test with the `reassoc` flag, I conditionally propagated the `FNegOp` FMF flag.

Personally I think _all_ cases in this function should be copying FMF from `FNegOp`, not from `I`. That is simple, correct (I think!) and useful.

In some cases you might be able to infer additional flags from `I` but that is much more subtle and (I assert!) far less useful. For example copying `nnan` from `I` is probably OK, but in that case it should be ORed into the flags from `FNegOp`.

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