fix corner case in unused (#3963)

fixes #3962

lib/compress.js

@@ -4521,7 +4521,7 @@ merge(Compressor.prototype, {
                         var index = indexOf_assign(sym, def);
                         if (index >= 0) assign_in_use[sym.id][index] = assign;
                         sym.eliminated++;
-                        return assign.transform(tt);
+                        return assign;
                     }));
                 } else if (head.length > 0 || tail.length > 0) {
                     node.definitions = head.concat(tail);