[LoopUnroll] NFC. Factor out runtime-loop.ll common test behavior.

Adding COMMON prefix to get common part handled there.
Needed to simplify test changes for D53440.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@345538 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed