[VPlan] Add & use VPValue for VPWidenGEPRecipe operands (NFC).

This patch adds VPValue version of the GEP's operands to
VPWidenGEPRecipe and uses them during code-generation.

Reviewers: Ayal, gilr, rengolin

Reviewed By: gilr

Differential Revision: https://reviews.llvm.org/D80220