[llvm] [AArch64][GlobalISel] Combine vecreduce(ext) to {U/S}ADDLV (PR #75832)

[David Green via llvm-commits]

https://github.com/davemgreen approved this pull request.

This LGTM from what I can tell, with a couple of extra suggestions. Can you pre-commit the tests separately too? Thanks

https://github.com/llvm/llvm-project/pull/75832