[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