[llvm] [X86] combineKSHIFT - fold kshiftr(kshiftr/extract_subvector(X, C1), C2) --> kshiftr(X, C1+C2) (PR #115528)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

@phoebewang I'd still like get this in, do you think this will just be neutral or actually cause regressions? 

https://github.com/llvm/llvm-project/pull/115528