[llvm] SelectionDAG/expandFMINNUM_FMAXNUM: skips vector if Op is legal for elements (PR #109570)

[Matt Arsenault via llvm-commits]

arsenm wrote:

We have really bad vector splitting logic if the vector type is legal. The correct vector splitting only happens by default for illegal vector types to narrower legal vector types in the vector legalization. Most legal expansions go straight to scalarization, which this also does. This is fine, but most of the other instances I see check based on which vector operations are legal, not that the scalar operation is legal. For consistency it's probably best to check the legality of the vector expansions 

https://github.com/llvm/llvm-project/pull/109570