[VPlan] Move recipe based VPlan generation to separate function.

This first step separates VPInstruction-based and VPRecipe-based
VPlan creation, which should make it easier to migrate to VPInstruction
based code-gen step by step.

Reviewers: Ayal, rengolin, dcaballe, hsaito, mkuper, mzolotukhin

Reviewed By: dcaballe

Subscribers: bollu, tschuett, rkruppe, llvm-commits

Differential Revision: https://reviews.llvm.org/D47477

llvm-svn: 334284