[SimplifyCFG] propagate fast-math-flags (FMF) from phi to select
This is another step towards having FMF apply only to FP values
rather than those + fcmp. See PR38086 for one of the original
And the test here is derived from PR39535:
Currently, we lose FMF when converting any phi to select in
SimplifyCFG. There are a small number of similar changes needed
to correct within SimplifyCFG, so it should be quick to patch
this pass up.