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

[Phoebe Wang via llvm-commits]

phoebewang wrote:

> The PR34592 -O0 regression is an unfortunate failure to cleanup with a later SimplifyDemandedElts pass like the -O3 does - I'm not sure how worried we should be tbh.

Is it due to FastISel fall back the SelectionDAG? I don't think it's a problem here, but I'm concerning the debuggability in general if we do such optimizations under O0.

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