[InstCombine] reorder FP select folds

There was a code comment about detecting min/max, and we were already
doing that later.

The real motivation is hinted at by the new TODO comment. I'm hoping
to untangle some FMF ambiguity in follow-on patches. See discussion
in issue #59279.

There are enough unknowns in FMF handling that I can't say with
certainty that this change is NFC, but it doesn't cause any existing
regression tests to change.