[LoopUnrollRuntime] Support multiple exit blocks unrolling when prolog remainder generated

With the NFC refactoring in rL307417 (git SHA 987dd01), all the logic
is in place to support multiple exit/exiting blocks when prolog
remainder is generated.
This patch removed the assert that multiple exit blocks unrolling is only
supported when epilog remainder is generated.

Also, added test runs and checks with PROLOG prefix in
runtime-loop-multiple-exits.ll test cases.

llvm-svn: 307435