[llvm] [VPlan] Detect and create partial reductions in VPlan. (NFCI) (PR #167851)

[Sam Tebbs via llvm-commits]

https://github.com/SamTebbs33 commented:

It's looking good and I agree that we should move towards creating partial reductions at the same time as bundling into `VPExpressionRecipe`.

https://github.com/llvm/llvm-project/pull/167851