[VPlan] Iterate over phi recipes to detect reductions to fix.

After refactoring the phi recipes, we can now iterate over all header
phis in a VPlan to detect reductions when it comes to fixing them up
when tail folding.

This reduces the coupling with the cost model & legal by using the
information directly available in VPlan. It also removes a call to
getOrAddVPValue, which references the original IR value which may
become outdated after VPlan transformations.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D100102