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 multiarr1;

void main()
{
    int[16][16] a;
    assert(a[0][0] == 0);
    assert(a[0][1] == 0);
    assert(a[0][2] == 0);
    assert(a[0][3] == 0);
    assert(a[10][13] == 0);
    assert(a[15][15] == 0);
    a[10][13] = 42;
    assert(a[0][0] == 0);
    assert(a[10][13] == 42);
    assert(a[15][15] == 0);
    {
        int* l = cast(int*)a;
        l += 10*16+13;
        assert(*l == 42);
    }
}