[LV] Remove the redundant using LoopVectorizationPlanner:VPlanPtr

VPlan.h already contains the declaration of VPlanPtr type alias:

using VPlanPtr = std::unique_ptr<VPlan>;

The LoopVectorizationPlanner class also contains the same declaration
of VPlanPtr and therefore LoopVectorize requires a long wording when
its methods return VPlanPtr:

    LoopVectorizationPlanner::VPlanPtr
    LoopVectorizationPlanner::buildVPlanWithVPRecipes(...)

but LoopVectorize.cpp includes VPlan.h (via LoopVectorizationPlanner.h)
and can use VPlanPtr from that header.

Patch by Pavel Samolysov.

Reviewers: hsaito, rengolin, fhahn

Reviewed By: fhahn

Differential Revision: https://reviews.llvm.org/D62576

llvm-svn: 362126