enhance reduce_vars (#1814)

- allow immediate assignment after declaration of variable
- relax modification rule for immutable value
- fix order of visit for TreeWalker
- remove extraneous code

lib/ast.js

@@ -182,21 +182,13 @@ var AST_BlockStatement = DEFNODE("BlockStatement", null, {
 }, AST_Block);
 
 var AST_EmptyStatement = DEFNODE("EmptyStatement", null, {
-    $documentation: "The empty statement (empty block or simply a semicolon)",
-    _walk: function(visitor) {
-        return visitor._visit(this);
-    }
+    $documentation: "The empty statement (empty block or simply a semicolon)"
 }, AST_Statement);
 
 var AST_StatementWithBody = DEFNODE("StatementWithBody", "body", {
     $documentation: "Base class for all statements that contain one nested body: `For`, `ForIn`, `Do`, `While`, `With`",
     $propdoc: {
         body: "[AST_Statement] the body; this should always be present, even if it's an AST_EmptyStatement"
-    },
-    _walk: function(visitor) {
-        return visitor._visit(this, function(){
-            this.body._walk(visitor);
-        });
     }
 }, AST_Statement);
 
@@ -551,11 +543,11 @@ var AST_Call = DEFNODE("Call", "expression args", {
     },
     _walk: function(visitor) {
         return visitor._visit(this, function(){
-            this.expression._walk(visitor);
             var args = this.args;
             for (var i = 0, len = args.length; i < len; i++) {
                 args[i]._walk(visitor);
             }
+            this.expression._walk(visitor);
         });
     }
 });