Undetected storage-related dependence with buffering in corner cases

[FabioLuporini]

When one uses buffering in a dummy Operator with a parallel time dimension there is a race condition due to OpenMP-izing the time loop.

Reproducer:

    grid = Grid(shape=(2, 2))
    time = grid.time_dim
    x, y = grid.dimensions

    usave = TimeFunction(name='usave', grid=grid, save=10)
    v = TimeFunction(name='v', grid=grid)

    eq = [Eq(v[time, x, y], usave)]

    op = Operator(eq, opt='buffering')

    trees = retrieve_iteration_tree(op)
    assert len(trees) == 2
    assert trees[1].root.dim is time
    assert not trees[1].root.is_Parallel
    assert trees[1].root.is_Sequential  # Obv

The issue is that buffering creates a storage-related dependence and what was previously discovered to be PARALLEL should now be downgraded to SEQUENTIAL