consolidate evaluate & reduce_vars (#1505)

- improve marking efficiency
- apply smarter `const` replacement to `var`

fixes #1501

test/compress/evaluate.js

@@ -615,7 +615,7 @@ call_args: {
         const a = 1;
         console.log(1);
         +function(a) {
-            return a;
+            return 1;
         }(1);
     }
 }