[VPlan] Representing backedge def-use feeding reduction phis.

This patch updates the code handling reduction recipes to also keep
track of the incoming value from the latch in the recipe. This is needed
to model the def-use chains completely in VPlan, so that it is possible
to replace the incoming value with an arbitrary VPValue.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D99294