[llvm] [VectorCombine] foldShuffleOfShuffles - fold "shuffle (shuffle x, undef), (shuffle y, undef)" -> "shuffle x, y" (PR #88743)

[David Green via llvm-commits]

https://github.com/davemgreen commented:

I have added a phase ordering test to show the vectorization around interleaving groups. It would be good to rebase and figure out why this is making things worse. I'm a bit sceptical of shuffle combines in general considering the likelihood that they can cause things to go wrong, but was hoping that adding the users would fix it. It looks like that already happens though, and another combine might be needed.

https://github.com/llvm/llvm-project/pull/88743