MDEV-25078: ALTER INDEX is inconsistent with ADD/DROP/RENAME index

Allowing ALTER KEY syntax in ALTER TABLE,so one can use:

ALTER TABLE tbl ALTER INDEX index_name IGNORED
ALTER TABLE tbl ALTER KEY index_name IGNORED

mysql-test/main/ignore_indexes.result

@@ -458,3 +458,24 @@ BEGIN
 SET IGNORED= a+b;
 END |
 ERROR HY000: Unknown system variable 'IGNORED'
+#
+# ALLOWING ALTER KEY syntax in ALTER TABLE
+#
+CREATE TABLE t1 (a INT, KEY (a));
+ALTER TABLE t1 ALTER INDEX a IGNORED;
+SHOW CREATE TABLE t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `a` int(11) DEFAULT NULL,
+  KEY `a` (`a`) IGNORED
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+DROP TABLE t1;
+CREATE TABLE t1 (a INT, KEY (a));
+ALTER TABLE t1 ALTER KEY a IGNORED;
+SHOW CREATE TABLE t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `a` int(11) DEFAULT NULL,
+  KEY `a` (`a`) IGNORED
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+DROP TABLE t1;

mysql-test/main/ignore_indexes.test

@@ -429,3 +429,16 @@ BEGIN
   SET IGNORED= a+b;
 END |
 DELIMITER ;|
+
+--echo #
+--echo # ALLOWING ALTER KEY syntax in ALTER TABLE
+--echo #
+
+CREATE TABLE t1 (a INT, KEY (a));
+ALTER TABLE t1 ALTER INDEX a IGNORED;
+SHOW CREATE TABLE t1;
+DROP TABLE t1;
+CREATE TABLE t1 (a INT, KEY (a));
+ALTER TABLE t1 ALTER KEY a IGNORED;
+SHOW CREATE TABLE t1;
+DROP TABLE t1;

sql/sql_yacc.yy

@@ -7794,7 +7794,7 @@ alter_list_item:
             if (unlikely(Lex->add_alter_list($4, $7, $3)))
               MYSQL_YYABORT;
           }
-        | ALTER INDEX_SYM ident ignorability
+        | ALTER key_or_index ident ignorability
           {
             LEX *lex= Lex;
             Alter_index_ignorability *ac= new (thd->mem_root)