[llvm] [AArch64] Lower scalable i1 vector add reduction to cntp (PR #100118)

[Max Beck-Jones via llvm-commits]

DevM-uk wrote:

Previously reviewed (with no further changes) in #99031. Reverted due to buildbot failures #99853.
Another patch which broke this batch was reverted in 3f8d77b.

https://github.com/llvm/llvm-project/pull/100118