[VPlan] Keep start value of VPWidenPHIRecipe as VPValue.
Similar to D92129, update VPWidenPHIRecipe to manage the start value as
VPValue. This allows adjusting the start value as a VPlan transform,
which will be used in a follow-up patch to support reductions during
Reviewed By: gilr
Differential Revision: https://reviews.llvm.org/D93975