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

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

@davemgreen if we can extend foldShuffleOfBinops to handle length changing shuffles, I think this will handle the cases you were seeing in #88693

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