[LV] NFC patch for moving VP*Recipe class definitions from LoopVectorize.cpp to VPlan.h

This is a small step forward to move VPlan stuff to where it should belong (i.e., VPlan.*):

  1. VP*Recipe classes in LoopVectorize.cpp are moved to VPlan.h.
  2. 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.
  3. InterleaveGroup class is moved from anonymous namespace to llvm namespace.
     Referencing it in anonymous namespace from VPlan.h ended up in warning.

Patch by Hideki Saito, thanks!

Differential Revision: https://reviews.llvm.org/D41045

llvm-svn: 320900