fix order of visit for TreeWalker

lib/ast.js

@@ -543,11 +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);
         });
     }
 });

test/compress/reduce_vars.js

@@ -2087,6 +2087,35 @@ var_assign_4: {
     }
 }
 
+var_assign_5: {
+    options = {
+        evaluate: true,
+        reduce_vars: true,
+        sequences: true,
+        side_effects: true,
+        unused: true,
+    }
+    input: {
+        !function() {
+            var a;
+            !function(b) {
+                a = 2;
+                console.log(a, b);
+            }(a);
+        }();
+    }
+    expect: {
+        !function() {
+            var a;
+            !function(b) {
+                a = 2,
+                console.log(a, b);
+            }(a);
+        }();
+    }
+    expect_stdout: "2 undefined"
+}
+
 immutable: {
     options = {
         evaluate: true,