Lower dmd CmpExp between classes to __cmp call

src/object.d

@@ -4653,17 +4653,33 @@ public import core.internal.switch_: __switch_error;
 public @trusted @nogc nothrow pure extern (C) void _d_delThrowable(scope Throwable);
 
 // Compare class and interface objects for ordering.
-private int __cmp(Obj)(Obj lhs, Obj rhs)
-if (is(Obj : Object))
+int __cmp(C1, C2)(C1 lhs, C2 rhs)
+if ((is(C1 : const(Object)) || (is(C1 == interface) && (__traits(getLinkage, C1) == "D"))) &&
+    (is(C2 : const(Object)) || (is(C2 == interface) && (__traits(getLinkage, C2) == "D"))))
 {
-    if (lhs is rhs)
+    static if (is(C1 == typeof(null)) && is(C2 == typeof(null)))
+    {
         return 0;
-    // Regard null references as always being "less than"
-    if (!lhs)
+    }
+    else static if (is(C1 == typeof(null)))
+    {
+        // Regard null references as always being "less than"
         return -1;
-    if (!rhs)
+    }
+    else static if (is(C2 == typeof(null)))
+    {
         return 1;
-    return lhs.opCmp(rhs);
+    }
+    else
+    {
+        if (lhs is rhs)
+            return 0;
+        if (lhs is null)
+            return -1;
+        if (rhs is null)
+            return 1;
+        return lhs.opCmp(rhs);
+    }
 }
 
 // objects