[VPlan] Add VPInstruction to VPRecipe transformation.

This patch introduces a VPInstructionToVPRecipe transformation, which
allows us to generate code for a VPInstruction based VPlan re-using the
existing infrastructure.

Reviewers: dcaballe, hsaito, mssimpso, hfinkel, rengolin, mkuper, javed.absar, sguggill

Reviewed By: dcaballe

Differential Revision: https://reviews.llvm.org/D46827

llvm-svn: 334969