[VPlan] Use VPDef for VPWidenGEPRecipe.

This patch turns updates VPWidenGEPRecipe to manage the value it defines
using VPDef. The VPValue is used  during VPlan construction and
codegeneration instead of the plain IR reference where possible.

Reviewed By: gilr

Differential Revision: https://reviews.llvm.org/D90561