[LoopInfo] Verify BBMap tracks innermost loops for BBs.

By checking that none of the child loops contain a BB we make sure BBMap
contains the innermost loop defining BB. This invariant was violated in
LoopInterchange and got caught by this assertion.

Reviewers: chandlerc, mzolotukhin, sanjoy, mehdi_amini, efriedma

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D45971

llvm-svn: 330698