[llvm] [AArch64] Fold uaddv(a) to a if the all lanes except the 0th are zeros (PR #159086)

[David Green via llvm-commits]

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

Thanks LGTM.

> Could this be generalized to other monoids:

It looks like due to the way we lower and/or (without sve) those can be handled OK already. The umax might need a similar change to this, it might depend on whether they come up in practice. Some of the reduces we could handle in IR, but these adds we only generate late during lowering ctpop.

https://github.com/llvm/llvm-project/pull/159086