[InstCombine] intersect FMF when reassociating FP min/max intrinsics

As discussed in PR45478:
https://bugs.llvm.org/show_bug.cgi?id=45478
...propagating FMF from the outer (second) call is not correct,
so intersect them instead.
I suspect we could do better (see TODO comment), but mismatched
FMF is probably too rare to care about.

Differential Revision: https://reviews.llvm.org/D78631