[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:

> @RKSimon How would pr120270 incorporated into this patch? I saw it being mentioned there

UniqueCstAmt provides a lot of data that this patch currently has to derive itself - it should be a lot of more straightforward to create a pair of shuffle permutation that packs/unpacks the matching shift amounts together.

I'd start with a simple iteration over the map and create the shuffle masks that pack the shifts into order, don't over complicate things.

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