[LoopBase] Strengthen isLoopExiting by requiring that BB must be inside the loop.

Currently isLoopExiting returns true for BBs that are not part of the
loop. To avoid hiding subtle bugs, this patch adds an assertion to make
sure the passed BB is inside the loop

Reviewers: reames, efriedma, hfinkel, arsenm, nhaehnle

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D63952

llvm-svn: 365077