improve unsafe evaluate of function (#2936)

Miscellaneous
- simplify `run_code()` hack

lib/compress.js

@@ -2344,7 +2344,7 @@ merge(Compressor.prototype, {
                 var fn = function() {};
                 fn.node = this;
                 fn.toString = function() {
-                    return this.node.print_to_string();
+                    return "function(){}";
                 };
                 return fn;
             }