[VPlan] Introduce VPLoopInfo analysis.

The patch introduces loop analysis (VPLoopInfo/VPLoop) for VPBlockBases.
This analysis will be necessary to perform some H-CFG transformations and
detect and introduce regions representing a loop in the H-CFG.

Reviewers: fhahn, rengolin, mkuper, hfinkel, mssimpso

Reviewed By: fhahn 

Differential Revision: https://reviews.llvm.org/D48816

llvm-svn: 338346