DAG: Improve fminimum/fmaximum vector expansion logic (#93579)

First, expandFMINIMUM_FMAXIMUM should be a never-fail API. The client
wanted it expanded, and it can always be expanded. This logic was tied
up with what the VectorLegalizer wanted.
    
Prefer using the min/max opcodes, and unrolling if we don't have a
vselect.
This seems to produce better code in all the changed tests.