[Polly] Check the properties of accesses to operands of a matrix-matrix

multiplication

The following code modifies elements of the array D.

    for (i = 0; i < _PB_NI; i++)
      for (j = 0; j < _PB_NJ; j++)
      {
        for (k = 0; k < _PB_NK; k++)
        {
          double Mul = A[i][k] * B[k][j];
          D[i][j][k] += Mul;
          C[i][j] += Mul;
        }
      }

Nevertheless, the code is recognised as a matrix-matrix multiplication, since
the second and third dimensions of D are accessed with non-zero strides.

This fixes the typo, which was made during the translation to C++ bindings
(https://reviews.llvm.org/D35845).

Reviewed By: Michael Kruse <llvm@meinersbur.de>

Differential Revision: https://reviews.llvm.org/D110491