[llvm] [X86][Codegen] Shuffle certain shifts on i8 vectors to create opportunity for vectorized shift instructions (PR #117980)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

@huangjd please can you regenerate the changed tests to get some ideas of what's going on so far? The cost of the shuffles has to be very low to be as cheap as pmullw. It could be that we might be better off just using the general vXi8 lowering with blendvb for some constant combos to get a similar effect to what you're trying here.

https://github.com/llvm/llvm-project/pull/117980