[llvm] [VPlan] Add ReductionStartVector VPInstruction. (PR #142290)

[Sam Tebbs via llvm-commits]

https://github.com/SamTebbs33 commented:

I think that ReductionStartVector will need to be handled in VPlanAnalysis::getVFScaleFactor so that its register usage isn't overestimated.

https://github.com/llvm/llvm-project/pull/142290