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

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

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

Thanks, I think for now we never try to get the scaling factor as it is produced outside the vector loop. I added an assert for now

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