`reuse_at` fails to create multiple buffers for different outputs

[hecmay]

Reduced test as followed. HeteroCL returns Segmentation Fault with no extra information.

    hcl.init()

    A = hcl.placeholder((10, 10))
    B = hcl.placeholder((10, 8))
    C = hcl.placeholder((8, 10))

    def kernel(a, b, c):

        @hcl.def_([(10,10), (10,8), (8,10)])
        def stencil(A, B, C):
            hcl.update(B, lambda y, x: A[y, x] + 2*A[y, x+1] + 3*A[y, x+2])
            hcl.update(C, lambda y, x: A[y, x] + 3*A[y+1, x] + 5*A[y+2, x])

        stencil(a, b, c)

    s = hcl.create_schedule([A, B, C], kernel)

    k = kernel.stencil
    RB1 = s.reuse_at(k.A, s[k], k.axis[1])
    RB2 = s.reuse_at(k.A, s[k], k.axis[0])
    f = hcl.build(s)

    np_A = np.random.randint(0, 10, size=(10, 10))
    np_B = np.zeros((10, 8), dtype="int")
    np_C = np.zeros((8, 10), dtype="int")

    for y in range(0, 10):
        for x in range(0, 8):
            np_B[y][x] = np_A[y][x]*1 + np_A[y][x+1]*2 + np_A[y][x+2]*3
            np_C[x][y] = np_A[x][y]*1 + np_A[x+1][y]*3 + np_A[x+2][y]*5

    hcl_A = hcl.asarray(np_A)
    hcl_B = hcl.asarray(np.zeros((10, 8), dtype="int"))
    hcl_C = hcl.asarray(np.zeros((8, 10), dtype="int"))

    print(hcl.lower(s))

    f(hcl_A, hcl_B, hcl_C)

    ret_B = hcl_B.asnumpy()
    ret_C = hcl_C.asnumpy()
    assert np.array_equal(np_B, ret_B)
    assert np.array_equal(np_C, ret_C)