minor clean-ups (#1600)

- remove obsolete optimisation in `AST_Binary` after #1477
- improve `TreeWalker.has_directive()` readability and resilience against multiple visits

lib/ast.js

@@ -984,8 +984,8 @@ TreeWalker.prototype = {
     push: function (node) {
         if (node instanceof AST_Lambda) {
             this.directives = Object.create(this.directives);
-        } else if (node instanceof AST_Directive) {
-            this.directives[node.value] = this.directives[node.value] ? "up" : true;
+        } else if (node instanceof AST_Directive && !this.directives[node.value]) {
+            this.directives[node.value] = node;
         }
         this.stack.push(node);
     },
@@ -1013,7 +1013,7 @@ TreeWalker.prototype = {
             for (var i = 0; i < node.body.length; ++i) {
                 var st = node.body[i];
                 if (!(st instanceof AST_Directive)) break;
-                if (st.value == type) return true;
+                if (st.value == type) return st;
             }
         }
     },