[llvm] DAG: Improve fminimum/fmaximum vector expansion logic (PR #93579)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> 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.


On top of #93550

https://github.com/llvm/llvm-project/pull/93579