[LoopInterchange] Check number of latch successors before accessing them.

In cases where the OuterMostLoopLatchBI only has a single successor,
accessing the second successor will fail.

This fixes a failure when building the test-suite with loop-interchange
enabled.

Reviewers: mcrosier, karthikthecool, davide

Reviewed by: karthikthecool

Differential Revision: https://reviews.llvm.org/D42906

llvm-svn: 324994