MDEV-17941 ALTER USER IF EXISTS does not work, although documentation says it should.

Mistake in syntax definition fixed - should be ALTER USER IF EXISTS,
not ALTER IF EXISTS USER.

mysql-test/r/alter_user.result

@@ -43,7 +43,7 @@ SET GLOBAL read_only = @start_read_only;
 alter user boo;
 ERROR HY000: Operation ALTER USER failed for 'boo'
 #--warning ER_CANNOT_USER
-alter if exists user boo;
+alter user if exists boo;
 Warnings:
 Error	1133	Can't find any matching row in the user table
 Note	1396	Operation ALTER USER failed for 'boo'

mysql-test/t/alter_user.test

@@ -41,7 +41,7 @@ SET GLOBAL read_only = @start_read_only;
 --error ER_CANNOT_USER
 alter user boo;
 --echo #--warning ER_CANNOT_USER
-alter if exists user boo;
+alter user if exists boo;
 
 --echo # Test password related altering.
 alter user foo identified by 'something';

sql/sql_yacc.yy

@@ -7294,10 +7294,10 @@ alter:
             lex->server_options.reset($3);
           } OPTIONS_SYM '(' server_options_list ')' { }
           /* ALTER USER foo is allowed for MySQL compatibility. */
-        | ALTER opt_if_exists USER_SYM clear_privileges grant_list
+        | ALTER USER_SYM opt_if_exists clear_privileges grant_list
           opt_require_clause opt_resource_options
           {
-            Lex->create_info.set($2);
+            Lex->create_info.set($3);
             Lex->sql_command= SQLCOM_ALTER_USER;
           }
         ;