[llvm] [InstCombine] [X86] pblendvb intrinsics must be replaced by select when possible (PR #137322)

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vortex73 wrote:

@RKSimon This is still a WIP, but I'd like any feedback on the approach so I can correct it right away. I've done a minor bit of refactoring. The replacement still doesn't seem to happen, I'm figuring out why. And as per earlier discussions I'm trying not to play around with OneUse checks.

https://github.com/llvm/llvm-project/pull/137322