[PATCH] don't unroll loops with zero trip count; fix for PR21409

[Sanjay Patel]

This change actually is in the runtime unroller...why we're getting here for a compile-time constant loop is another bug?

In the compile-time unroller, we do this check:
    TripCount = SE->getSmallConstantTripCount(L, ExitingBlock);

And getSmallConstantTripCount() has this:
  // In case of integer overflow, this returns 0, which is correct.
  return ((unsigned)ExitConst->getZExtValue()) + 1;

http://reviews.llvm.org/D6200