[LoopDeletion] (cleanup, NFC) Make this test actually test what it

claims to test.

LoopSimplify was unifying the multiple exits in this test case, making
it never even test the multiple exit handling of LoopDeletion. Doh.

Now it works (thanks to a great idea from mkuper) and will fail if we
ever change something to make it stop working.

llvm-svn: 292331