[LV][VPlan] Build plain CFG with simple VPInstructions for outer loops.

Patch #3 from VPlan Outer Loop Vectorization Patch Series #1
(RFC: http://lists.llvm.org/pipermail/llvm-dev/2017-December/119523.html).

Expected to be NFC for the current inner loop vectorization path. It
introduces the basic algorithm to build the VPlan plain CFG (single-level
CFG, no hierarchical CFG (H-CFG), yet) in the VPlan-native vectorization
path using VPInstructions. It includes:
  - VPlanHCFGBuilder: Main class to build the VPlan H-CFG (plain CFG without nested regions, for now).
  - VPlanVerifier: Main class with utilities to check the consistency of a H-CFG.
  - VPlanBlockUtils: Main class with utilities to manipulate VPBlockBases in VPlan.

Reviewers: rengolin, fhahn, mkuper, mssimpso, a.elovikov, hfinkel, aprantl.

Differential Revision: https://reviews.llvm.org/D44338

llvm-svn: 332654