MDEV-32986 Make regexp operator work in spider group by handler

In spider_db_mbase_util::print_item_func(), if the sql item_func has
an UNKNOWN_FUNC type, by default the spider group by handler (gbh)
transform infix to prefix. But regexp should remain infix, so we add
an if condition to account for this.

storage/spider/mysql-test/spider/bugfix/r/mdev_32986.result

+#
+# MDEV-32907
+#
+for master_1
+for child2
+for child3
+CREATE SERVER srv FOREIGN DATA WRAPPER mysql
+OPTIONS (SOCKET "$MASTER_1_MYSOCK", DATABASE 'test',user 'root');
+create table t2 (c varchar(16));
+create table t1 (c varchar(16)) ENGINE=Spider
+COMMENT='WRAPPER "mysql", srv "srv",TABLE "t2"';
+insert into t1 values ('TestSpiderRegex');
+select c from t1 where c regexp '(Test|Spider|Regex)';
+c
+TestSpiderRegex
+drop table t1, t2;
+drop server srv;
+for master_1
+for child2
+for child3
+#
+# end of test mdev_32907
+#

storage/spider/mysql-test/spider/bugfix/t/mdev_32986.test

+--echo #
+--echo # MDEV-32907
+--echo #
+--disable_query_log
+--disable_result_log
+--source ../../t/test_init.inc
+--enable_result_log
+--enable_query_log
+evalp CREATE SERVER srv FOREIGN DATA WRAPPER mysql
+OPTIONS (SOCKET "$MASTER_1_MYSOCK", DATABASE 'test',user 'root');
+
+create table t2 (c varchar(16));
+create table t1 (c varchar(16)) ENGINE=Spider
+COMMENT='WRAPPER "mysql", srv "srv",TABLE "t2"';
+
+insert into t1 values ('TestSpiderRegex');
+select c from t1 where c regexp '(Test|Spider|Regex)';
+drop table t1, t2;
+drop server srv;
+
+--disable_query_log
+--disable_result_log
+--source ../../t/test_deinit.inc
+--enable_result_log
+--enable_query_log
+--echo #
+--echo # end of test mdev_32907
+--echo #

storage/spider/spd_db_mysql.cc

@@ -5856,12 +5856,17 @@ int spider_db_mbase_util::print_item_func(
           item_count -= 2;
           break;
         }
-      } else if (func_name_length == 6 &&
-        !strncasecmp("istrue", func_name, func_name_length)
-      ) {
-        last_str = SPIDER_SQL_IS_TRUE_STR;
-        last_str_length = SPIDER_SQL_IS_TRUE_LEN;
-        break;
+      } else if (func_name_length == 6)
+      {
+        if (!strncasecmp("istrue", func_name, func_name_length))
+        {
+          last_str= SPIDER_SQL_IS_TRUE_STR;
+          last_str_length= SPIDER_SQL_IS_TRUE_LEN;
+          break;
+        }
+        else if (!strncasecmp("regexp", func_name, func_name_length))
+          /* Keep the infix expression */
+          break;
       } else if (func_name_length == 7)
       {
         if (!strncasecmp("isfalse", func_name, func_name_length))