Context: Patch Series #1 for outer loop vectorization support in LV
using VPlan. (RFC: http://lists.llvm.org/pipermail/llvm-dev/2017-December/119523.html).
This patch introduces a plain CFG simplification step during H-CFG
construction. This simplification is aimed at preparing the CFG to
introduce VPLoopRegions in the H-CFG (subsequent patch, working on it).
It consists of splitting some VPBasicBlocks that otherwise would belong to
more than one region.