[llvm] [VP][DAGCombine] Use `simplifySelect` when combining vp.select. (PR #87342)

[via llvm-commits]

AinsleySnow wrote:

I addressed the comments in the newly-pushed commits.

I added pre-commit test cases to demonstrate that the tests will hit the DAG combine. Before we use `simplifySelect`, there are redundant `vmand`/`vmor` in the generated assembly code but after that they turn into single `vmv1r.v`s.

Thank you for your suggestions!

https://github.com/llvm/llvm-project/pull/87342