fix side_effects on AST_Expansion (#2165)

fixes #2163

lib/compress.js

@@ -2672,6 +2672,9 @@ merge(Compressor.prototype, {
             if (expr) merge_sequence(expressions, expr);
             return make_sequence(this, expressions);
         });
+        def(AST_Expansion, function(compressor, first_in_statement){
+            return this.expression.drop_side_effect_free(compressor, first_in_statement);
+        });
     })(function(node, func){
         node.DEFMETHOD("drop_side_effect_free", func);
     });