enhance reduce_vars for AST_Accessor (#2339)

fixes #2336

lib/compress.js

@@ -285,6 +285,10 @@ merge(Compressor.prototype, {
 
     AST_Node.DEFMETHOD("reset_opt_flags", function(compressor, rescan) {
         var reduce_vars = rescan && compressor.option("reduce_vars");
+        // Stack of look-up tables to keep track of whether a `SymbolDef` has been
+        // properly assigned before use:
+        // - `push()` & `pop()` when visiting conditional branches
+        // - backup & restore via `save_ids` when visiting out-of-order sections
         var safe_ids = Object.create(null);
         var suppressor = new TreeWalker(function(node) {
             if (!(node instanceof AST_Symbol)) return;
@@ -391,10 +395,9 @@ merge(Compressor.prototype, {
                     return true;
                 }
                 if (node instanceof AST_Accessor) {
-                    var save_ids = safe_ids;
-                    safe_ids = Object.create(null);
+                    push();
                     descend();
-                    safe_ids = save_ids;
+                    pop();
                     return true;
                 }
                 if (node instanceof AST_Binary