slice of MatrixSlices simplify

Reduces slices of slices

```Python
    >>> from sympy import MatrixSymbol
    >>> X = MatrixSymbol('X', 10, 10)
    >>> X[:, 1:5][5:8, :]
    X[5:8, 1:5]
    >>> X[1:9:2, 2:6][1:3, 2]
    X[3:7:2, 4]
```