fix slowness in unused for blocks (#2614)

fixes #2609

lib/compress.js

@@ -2827,6 +2827,7 @@ merge(Compressor.prototype, {
                     if (in_list && all(node.body, can_be_evicted_from_block)) {
                         return MAP.splice(node.body);
                     }
+                    return node;
                 }
                 if (node instanceof AST_Scope) {
                     var save_scope = scope;