[LoopUnrollRuntime] Bailout when multiple exiting blocks to the unique latch exit block

Currently, we do not support multiple exiting blocks to the
latch exit block. However, this bailout wasn't triggered when we had a
unique exit block (which is the latch exit), with multiple exiting
blocks to that unique exit.

Moved the bailout so that it's triggered in both cases and added
testcase.

llvm-svn: 307291