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

[Sam Tebbs via llvm-commits]

SamTebbs33 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.

I don't think there will be many difficult-to-resolve conflicts when rebasing https://github.com/llvm/llvm-project/pull/144908 and https://github.com/llvm/llvm-project/pull/144281 on top of this change since most of this PR affects the early partial reduction detection code. I'm happy to fix the merge conflicts in `VPPartialReductionRecipe::computeCost` when I rebase #144908 on top of this.

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