[X86][Reduce] Preserve fast math flags when change it. NFCI

@arsenm raised a good question that we should use a flag guard.
But I found it is not a problem as long as user uses intrinsics only: https://godbolt.org/z/WoYsqqjh3
Anyway, it is still nice to have.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D140467