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 S {
    int a; int b; int c; int d = 7;
}
void test(int i) {
    S s = { 1, i };   // q.a = 1, q.b = i, q.c = 0, q.d = 7
    assert(s.a == 1);
    assert(s.b == i);
    assert(s.c == 0); // line 8
    assert(s.d == 7);
}
void main() {
    test(42);
}