[LoopInfo] Fix getUniqueNonLatchExitBlocks

It is possible that exit block has two predecessors and one of them is a latch
block while another is not.

Current algorithm is based on the assumption that all exits are dedicated
and therefore we can check only first predecessor of loop exit to find all unique
exits.

However if we do not consider latch block and it is first predecessor of some
exit then this exit will be found.

Regression test is added.

As a side effect of algorithm re-writing, the restriction that all exits are dedicated
is eliminated.

Reviewers: reames, fhahn, efriedma
Reviewed By: efriedma
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D64787

llvm-svn: 366294