GPU suggestions: Enforce perfectly nested loops for collapse clauses

[lukasrothenberger]

Collapse clauses may only be suggested if the included loops are perfectly nested (i.e. no further statements are contained in a outer loop).

#pragma omp target teams distribute parallel for collapse(2) map(tofrom: a,v,v_out) 
  for (i = 0; i < N; i++) {
    for (j = 0; j < N; j++) {
        a[i][j] = i * j + 0.01;
    }
    v_out[i] = i * j + 0.01;
    v[i] = i * j + 0.01;
  }