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

[Karl Meakin via llvm-commits]

Kmeakin wrote:

Could this be generalized to other monoids:
* or(x, 0) => x
* And(x, -1) => x
* Umax(x, 0) => x
etc

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