This is a minimal start to correcting a problem most directly discussed in PR38086:
We have been hacking around a limitation for FP select patterns by using the fast-math-flags on the condition of the select rather than the select itself.
This patch just allows FMF to appear with the 'select' opcode. No changes are needed to "FPMathOperator" because it already includes select-of-FP because that definition is based on the return value type.
Once we have this ability, we can start correcting and adding IR transforms to use the FMF on a 'select' instruction. The instcombine and vectorizer test diffs only show that the IRBuilder change is behaving as expected by applying an FMF guard value to 'select'.