[llvm] [LV] Add support for partial reductions without a binary op (PR #133922)

[Paul Walker via llvm-commits]

paulwalker-arm wrote:

> > We at Arm have discussed this offline and because this PR improves a key workload and the LLVM 21 branch point is fast approaching we prefer to detach it from the refactoring work Sander mentions above. There is still potential that all PRs could land in time but we just don't want to risk this PR not making the release.
> > @fhahn: Any objections to us prioritising the performance improvement?
> 
> I agree with @sdesmalen-arm that it probably would be good to first land the refactorings if possible. But if it looks like the refactorings won't land in time by the middle of next week, lets pull this PR ahead of the other patches? That should hopefully leave enough time before the branch in either case.

Works for me.  Let's say Wednesday 12pm uk time.  That should give us just enough time to recover if post-merge testing uncovers something unexpected.

https://github.com/llvm/llvm-project/pull/133922