[llvm] [InstCombine] Move extends across identity shuffles. (PR #146901)

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

ping :)

Is moving the extends across identity shuffles OK as a start? If not, I could also move it back to VectorCombine, although the general InstCombine version gives better results and triggers in more cases on a number of workloads using AArch64 intrinsics.

https://github.com/llvm/llvm-project/pull/146901