[llvm] Reapply "[SelectionDAG] Recurse through mask expression trees in WidenVSELECTMask" (PR #217307)

[Valeriy Savchenko via llvm-commits]

SavchenkoValeriy wrote:

The original PR tried to juggle element type widening with element count change and was prone to errors. In the new version, I deliberately split these two into two separate concerns.

https://github.com/llvm/llvm-project/pull/217307