Instant DDL downgrades the MDL when table is empty

diff --git a/sql/sql_table.cc b/sql/sql_table.cc

index 03df92cd484..eb57242edbe 100644

--- a/sql/sql_table.cc

+++ b/sql/sql_table.cc

@@ -7258,16 +7258,15 @@ static bool mysql_inplace_alter_table(THD *thd,

     lock for prepare phase under LOCK TABLES in the same way as when

     exclusive lock is required for duration of the whole statement.

   */

-  if (!ha_alter_info->mdl_exclusive_after_prepare &&

-      (inplace_supported == HA_ALTER_INPLACE_EXCLUSIVE_LOCK ||

-       ((inplace_supported == HA_ALTER_INPLACE_COPY_NO_LOCK ||

+  if (inplace_supported == HA_ALTER_INPLACE_EXCLUSIVE_LOCK ||

+      ((inplace_supported == HA_ALTER_INPLACE_COPY_NO_LOCK ||

         inplace_supported == HA_ALTER_INPLACE_COPY_LOCK ||

         inplace_supported == HA_ALTER_INPLACE_NOCOPY_NO_LOCK ||

         inplace_supported == HA_ALTER_INPLACE_NOCOPY_LOCK ||

         inplace_supported == HA_ALTER_INPLACE_INSTANT) &&

        (thd->locked_tables_mode == LTM_LOCK_TABLES ||

         thd->locked_tables_mode == LTM_PRELOCKED_UNDER_LOCK_TABLES)) ||

-      alter_info->requested_lock == Alter_info::ALTER_TABLE_LOCK_EXCLUSIVE))

+      alter_info->requested_lock == Alter_info::ALTER_TABLE_LOCK_EXCLUSIVE)

   {

     if (wait_while_table_is_used(thd, table, HA_EXTRA_FORCE_REOPEN))

       goto cleanup;

@@ -7379,7 +7378,8 @@ static bool mysql_inplace_alter_table(THD *thd,

     necessary only for prepare phase (unless we are not under LOCK TABLES) and

     user has not explicitly requested exclusive lock.

   */

-  if ((inplace_supported == HA_ALTER_INPLACE_COPY_NO_LOCK ||

+  if (!ha_alter_info->mdl_exclusive_after_prepare &&

+      (inplace_supported == HA_ALTER_INPLACE_COPY_NO_LOCK ||

        inplace_supported == HA_ALTER_INPLACE_COPY_LOCK ||

        inplace_supported == HA_ALTER_INPLACE_NOCOPY_LOCK ||

        inplace_supported == HA_ALTER_INPLACE_NOCOPY_NO_LOCK) &&