Dependences: Do not fail in case a schedule eliminates all dependences

The following example shows a non-parallel loop

void f(int a[]) {
  int i;
  for (i = 0; i < 10; ++i)
    A[i] = A[i+5];
}

which, in case we import a schedule that limits the iteration domain
to 0 <= i < 5, becomes parallel. Previously we crashed in such cases, now we
just recognize it as parallel.

This fixes http://llvm.org/PR19435

Reported-by: Jeremy Huddleston Sequoia <jeremyhu@apple.com>