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)

struct bar {
    int bar;
}

void main() {
    bar Bar;
    with (Bar)
    {
        assert(Bar.bar == 0);
        void test()
        {
            bar ++;
        }
        test();
    }
    assert(Bar.bar == 1);
}