[llvm] [SelectionDAG] Recurse through mask expression trees in WidenVSELECTMask (PR #188085)

[Valeriy Savchenko via llvm-commits]

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SavchenkoValeriy wrote:

I wasn't able to come up with an example that will force us to change the element count, and thus prevent us from using the same shuffle mask.

https://github.com/llvm/llvm-project/pull/188085