[llvm] [X86] optimize saturating (masked) pack (PR #169995)

[Simon Pilgrim via llvm-commits]

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

I've managed to add all your tests to trunk now, so please can you fix the merge conflicts and we should be able to see the codegen diffs

https://github.com/llvm/llvm-project/pull/169995