[llvm] [VectorCombine] foldShuffleOfBinops - add support for length changing shuffles (PR #88899)

[David Green via llvm-commits]

https://github.com/davemgreen commented:

Thanks for adding a cost-model to this transform. That was going to be on my list of things to add. (Unfortunately there is already a transform in instcombine that isn't costed can mess up patterns already). The cases I've seen of this making things worse have either been to do with cases that were already going wrong getting a little worse, or commuting the operands making the transform harder to undo later. I don't have a lot of tests for shuffles from intrinsics.

https://github.com/llvm/llvm-project/pull/88899