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)

class C
{
    int c;
    long f(long l)
    {
        return l;
    }
}

class D : C
{
    int d;
    override long f(long l)
    {
        return l*2;
    }
}

void main()
{
    scope c = new C;
    assert(c.f(25L) == 25);
    scope d = new D;
    assert(d.f(25L) == 50);
    C cd = d;
    assert(cd.f(25L) == 50);
    assert(func(d,25L) == 50);
}

long func(C c, long l)
{
    return c.f(l);
}