[llvm] [SelectionDAG] Move VSelect sign pattern check from AArch64 to general SelectionDAG (PR #151840)

[via llvm-commits]

AZero13 wrote:

> > For some reason the check is already there, but it bails out. Probably to allow aarch64 to transform it, but why not have the logic in SelectionDAG then?
> 
> Have you consulted git blame and git log to see if you can find an answer?

I did actually. 

https://github.com/llvm/llvm-project/pull/151840