[VPlan] Manage scalarized values using VPValues.

This patch updates codegen to use VPValues to manage the generated
scalarized instructions.

Reviewed By: gilr

Differential Revision: https://reviews.llvm.org/D92285

GitOrigin-RevId: 54a14c264a245ae31e40581ab21be2ca5b6b1962
6 files changed