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 bug21;
extern(C) int printf(char*, ...);

void main()
{
    int i = 42;
    auto a = {
        int j = i*2;
        auto b = {
            return j;
        };
        return b();
    };
    int i2 = a();
    printf("%d\n", i2);
    assert(i2 == i*2);
}