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

[Valeriy Savchenko via llvm-commits]

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SavchenkoValeriy wrote:

All the operations that we traverse should have the same element counts by construction and I removed the element widening from SetCC handling. Thinking again about it, it might cause some issues and I'm trying to find a corner case. So far everything that I've tried ends up being correct due to a separate legalization of the element counts.

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