[llvm] [X86] Fold BLEND(PERMUTE(X), PERMUTE(Y)) -> PERMUTE(BLEND(X, Y)) (PR #90219)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

@goldsteinn I've pulled the combine out into its own combineBlendOfPermutes(). I've managed to get vXi16 VPBLENDW support working, with a few restrictions to avoid infinite combine loops due to creating more shuffles than we started with.....

It should be a lot easier to add BLENDV/VSELECT support in a later PR now.

https://github.com/llvm/llvm-project/pull/90219