[llvm] [ValueTracking][X86] Compute KnownBits for phadd/phsub (PR #92429)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon requested changes to this pull request.

It might be better to focus on the SelectionDAG variant first to get this right, but its up to you.

https://github.com/llvm/llvm-project/pull/92429