[LoopInfo] Add verification by recomputation.

Summary:
Current implementation of LI verifier isn't ideal and fails to detect
some cases when LI is incorrect. For instance, it checks that all
recorded loops are in a correct form, but it has no way to check if
there are no more other (unrecorded in LI) loops in the function. This
patch adds a way to detect such bugs.

Reviewers: chandlerc, sanjoy, hfinkel

Subscribers: llvm-commits, silvas, mzolotukhin

Differential Revision: https://reviews.llvm.org/D23437

llvm-svn: 280280