Implement first D-specific optimization pass: -simplify-drtcalls. It uses the machinery of the standard -simplify-libcalls pass, but optimizes calls to the D runtime instead of calls to C libraries. At the moment, these optimizations are implemented by this pass: - Avoid the runtime call for `arr.length = newlen` if it can determine that the new length isn't longer than the old one. - Ditto for `cast(T[]) arr` if it will clearly always succeed. (e.g. if the length of the original array is zero, or if the old element size is a multiple of the new element size)

module multiarr3;

void main()
{
    static int[2][2] arr = [[1,2],[3,4]];
    assert(arr[0][0] == 1);
    assert(arr[0][1] == 2);
    assert(arr[1][0] == 3);
    assert(arr[1][1] == 4);
}