minor tweaks (#5672)

lib/compress.js

@@ -11607,11 +11607,15 @@ Compressor.prototype.compress = function(node) {
                         right: right.right,
                     }),
                 });
-                left.fixed = function() {
-                    return assign.right;
-                };
-                left.fixed.assigns = [ assign ];
-                left.definition().references.push(left);
+                if (left.fixed) {
+                    left.fixed = function() {
+                        return assign.right;
+                    };
+                    left.fixed.assigns = [ assign ];
+                }
+                var def = left.definition();
+                def.references.push(left);
+                def.replaced++;
                 return assign.optimize(compressor);
             }
         }