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

[via llvm-commits]

SahilPatidar wrote:

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

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