enhance unused (#4129)

lib/compress.js

@@ -4747,6 +4747,13 @@ merge(Compressor.prototype, {
                             }));
                         }
                     }
+                } else if (node instanceof AST_UnaryPostfix
+                    && node.expression instanceof AST_SymbolRef
+                    && indexOf_assign(node.expression.definition(), node) < 0) {
+                    return make_node(AST_UnaryPrefix, node, {
+                        operator: "+",
+                        expression: node.expression
+                    });
                 }
             }
             if (node instanceof AST_Call) calls_to_drop_args.push(node);