[LV][VPlan] Refactor VPReductionRecipe to use reference for member RdxDesc

This commit refactors the implementation of VPReductionRecipe to use
reference instead of pointer for member RdxDesc. Because the member
RdxDesc in VPReductionRecipe should not be a nullptr, using a reference
will provide clearer semantics.

Reviewed By: fhahn

Differential Revision: https://reviews.llvm.org/D158058