[VPlan] Move recipe construction to VPRecipeBuilder.

This patch moves the recipe-creation functions out of
LoopVectorizationPlanner, which should do the high-level
orchestration of the transformations.

Reviewers: dcaballe, rengolin, hsaito, Ayal

Reviewed By: dcaballe

Differential Revision: https://reviews.llvm.org/D47595

llvm-svn: 334305