Return conservative result in case the dependence check timed out

For complex examples it may happen that we do not compute dependences. In this
case we do not want to crash, but just not detect parallel loops.

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@204470 91177308-0d34-0410-b5e6-96231b3b80d8