enhance side_effects (#4675)

lib/compress.js

@@ -8516,6 +8516,8 @@ merge(Compressor.prototype, {
                             }
                         });
                         return node;
+                    } else if (!node.has_side_effects(compressor)) {
+                        self.drop_side_effect_free = return_null;
                     }
                 }
                 var arg_used, insert, in_loop, scope;