[llvm] [VectorCombine] Add foldShuffleToIdentity (PR #88693)

[David Green via llvm-commits]

davemgreen wrote:

> Final question - do we need handling for DivRem instructions to make sure we don't introduce poison mask elements? see [4cc9c6d](https://github.com/llvm/llvm-project/commit/4cc9c6d98dfef90d1ffa69977d13ffa2894a10f7)

Thanks - I hadn't considered div/rem as they don't as often come up for vectors. I can exclude them for now if that sounds OK, but as far as I understand they might be fine as we end up removing the shuffles so do not have poison lanes left over.
https://alive2.llvm.org/ce/z/NAyQty

https://github.com/llvm/llvm-project/pull/88693