add comments for #1602

lib/compress.js

@@ -151,7 +151,17 @@ merge(Compressor.prototype, {
             node = node.hoist_declarations(this);
             was_scope = true;
         }
+        // Before https://github.com/mishoo/UglifyJS2/pull/1602 AST_Node.optimize()
+        // would call AST_Node.transform() if a different instance of AST_Node is
+        // produced after OPT().
+        // This corrupts TreeWalker.stack, which cause AST look-ups to malfunction.
+        // Migrate and defer all children's AST_Node.transform() to below, which
+        // will now happen after this parent AST_Node has been properly substituted
+        // thus gives a consistent AST snapshot.
         descend(node, this);
+        // Existing code relies on how AST_Node.optimize() worked, and omitting the
+        // following replacement call would result in degraded efficiency of both
+        // output and performance.
         descend(node, this);
         var opt = node.optimize(this);
         if (was_scope && opt instanceof AST_Scope) {