This is a small step forward to move VPlan stuff to where it should belong (i.e., VPlan.*) --- easy and obvious things at a time, in small increment.
- VP*Recipe classes in LoopVectorize.cpp are moved to VPlan.h.
- Many of VP*Recipe::print() and execute() definitions are still left in LoopVectorize.cpp since they refer to things declared in LoopVectorize.cpp. To be moved to VPlan.cpp at a later time.
- InterleaveGroup class is moved from anonymous namespace to llvm namespace. Referencing it in anonymous namespace from VPlan.h ended up in warning.