[PhaseOrdering] Add test for missed vectorization with vector::at calls.

This test illustrates missed vectorization of loops with multiple
std::vector::at calls, like

    int sum(std::vector<int> *A, std::vector<int> *B, int N) {
      int cost = 0;
      for (int i = 0; i < N; ++i)
        cost += A->at(i) + B->at(i);
      return cost;
    }

https://clang.godbolt.org/z/KbYoaPhvq