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

[David Green via llvm-commits]

https://github.com/davemgreen commented:

> Do you intend to support intrinsics at some point? I notice you have a abs test

Yep, those and a few other operations. I wanted to make sure the basics were working before adding the others but they are hopefully relatively straight forward.

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