[VPlan] Add VPReductionPHIRecipe (NFC).

This patch is a first step towards splitting up VPWidenPHIRecipe into
separate recipes for the 3 distinct cases they model:

    1. reduction phis,
    2. first-order recurrence phis,
    3. pointer induction phis.

This allows untangling the code generation and allows us to reduce the
reliance on LoopVectorizationCostModel during VPlan code generation.

Discussed/suggested in D100102, D100113, D104197.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D104989