improve unsafe_Func (#2171)

- minimise disturbance to `compute_char_frequency()`
- remove extraneous quotation marks

test/compress/functions.js

@@ -265,7 +265,7 @@ issue_203: {
     }
     expect: {
         var m = {};
-        var fn = Function("n", "o", "o.exports=42");
+        var fn = Function("n,o", "o.exports=42");
         fn(null, m, m.exports);
         console.log(m.exports);
     }