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

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

Yes - we've never come up with a good way to accumulate candidate folds and only commit them if/when it becomes cost effective.

Please don't think I want to just take #88743 + #88899 instead of this, I think they're both needed, I just want to make sure we don't end up with a heavy fold constantly doing too much of the combining.

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