[LoopInterchange] Use getExitBlock()/getExitingBlock instead of manual impl.

This also means we have to check if the latch is the exiting block now,
as `transform` expects the latches to be the exiting blocks too.

https://bugs.llvm.org/show_bug.cgi?id=36586

Reviewers: efriedma, davide, karthikthecool

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D45279

llvm-svn: 330806