[LoopUnrollRuntime] NFC: Refactored safety checks of unrolling multi-exit loop

Refactored the code and separated out a function
`canSafelyUnrollMultiExitLoop` to reduce redundant checks and make it
easier to add profitability heuristics later.
Added tests to runtime unrolling to make sure that unrolling for
multi-exit loops is not done unless the option
-unroll-runtime-multi-exit is true.

llvm-svn: 307843