[llvm] [VectorCombine] Add free concats to shuffleToIdentity. (PR #94954)

[David Green via llvm-commits]

https://github.com/davemgreen commented:

Thanks for taking a look. The cost-model issue I have not heard back from, so I believe / am hoping it was fixed by one of the backend patches.

https://github.com/llvm/llvm-project/pull/94954