[llvm] [SelectionDAG] Remove UnsafeFPMath check in `visitFADDForFMACombine` (PR #127770)

[via llvm-commits]

paperchalice wrote:

FMA instructions`AMDGPU/fneg-combines.f16.ll`, `AMDGPU/fneg-combines.new.ll` and `X86/sqrt-fastmath.ll` are now unoptimized due to lack of "unsafe-fp-math" attribute...

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The command line option `--enable-unsafe-fp-math` is useless in CodeGen tests after a certain time point 
Need to find time to clean them out.

https://github.com/llvm/llvm-project/pull/127770