[LV][VPlan] NFC patch to move LoopVectorizationPlanner class out of LoopVectorize.cpp

Another small step forward to move VPlan stuff outside of LoopVectorize.cpp.

VPlanBuilder.h is renamed to LoopVectorizationPlanner.h
LoopVectorizationPlanner class is moved from LoopVectorize.cpp to
LoopVectorizationPlanner.h LoopVectorizationCostModel::VectorizationFactor
class is moved to LoopVectorizationPlanner.h (used by the planner class) ---
this needs further streamlining work in later patches and thus all I did was
take it out of the CostModel class and moved to the header file.  The callback
function had to stay inside LoopVectorize.cpp since it calls an
InnerLoopVectorizer member function declared in it.  Next Steps: Make
InnerLoopVectorizer, LoopVectorizationCostModel, and other classes more modular
and more aligned with VPlan direction, in small increments.

Previous step was: r320900 (https://reviews.llvm.org/D41045)

Patch by Hideki Saito, thanks!

Differential Revision: https://reviews.llvm.org/D41420

llvm-svn: 321962