[llvm] [X86][AVX] Prefer VPSRAV to VPSRA style shifts for known splats #39424 (PR #87913)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon commented:

Most of those test changes will probably disappear with the logic fix - we might have to add extra test coverage (but get the fix in first)

https://github.com/llvm/llvm-project/pull/87913