[VPlan] Move reduction start value creation to widenPHIRecipe.

This was suggested to prepare for D93975.

By moving the start value creation to widenPHInstruction, we set the
stage to manage the start value directly in VPWidenPHIRecipe, which be
used subsequently to set the 'resume' value for reductions during
epilogue vectorization.

It also moves RdxDesc to the recipe, so we do not have to rely on Legal
to look it up later.

Reviewed By: gilr

Differential Revision: https://reviews.llvm.org/D94175