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

[via llvm-commits]

Zentrik wrote:

Hi @davemgreen, I'm seeing an assertion failure in `foldShuffleToIdentity` at https://github.com/llvm/llvm-project/issues/110382. I suspect this is due to the `auto *Ty = dyn_cast<FixedVectorType>(I.getType());` at the beginning of the function but not sure. Could you take a look please?

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