MDEV-15507 Assertion failed in dict_check_sys_tables on upgrade from 5.5

The InnoDB system table column SYS_TABLES.MIX_LEN was repurposed
in InnoDB Plugin for MySQL 5.1, in
commit 91111174 (MySQL 5.1.46).
Until MySQL 5.6, it only contained a flag DICT_TF2_TEMPORARY.

MySQL 5.6 introduced a number of flags that were transient
in nature. One of these was introduced in 5.6.5, originally
called DICT_TF2_USE_TABLESPACE and later renamed to
DICT_TF2_USE_FILE_PER_TABLE. MySQL 5.7.6 introduced logic
that insists that the flag be set for any table that does not
reside in a shared tablespace, breaking upgrade from MySQL 5.5.

MariaDB does not support shared tablespaces other than the
InnoDB system tablespace. Also, some dependencies on
SYS_TABLES.MIX_LEN were removed in an earlier fix:
MDEV-13084 MariaDB 10.2 crashes on corrupted SYS_TABLES.MIX_LEN field
(commit e813fe86).

dict_check_sys_tables(): Remove a bogus debug assertion, and add a
comment that explains how DICT_TF2_USE_FILE_PER_TABLE is used.

dict_table_is_file_per_table(): Remove a bogus debug assertion.

storage/innobase/dict/dict0load.cc

@@ -1436,12 +1436,13 @@ dict_check_sys_tables(
 			continue;
 		}
 
-		/* If the table is not a predefined tablespace then it must
-		be in a file-per-table tablespace.
-		Note that flags2 is not available for REDUNDANT tables,
-		so don't check those. */
-		ut_ad(!DICT_TF_GET_COMPACT(flags)
-		      || flags2 & DICT_TF2_USE_FILE_PER_TABLE);
+		/* For tables or partitions using .ibd files, the flag
+		DICT_TF2_USE_FILE_PER_TABLE was not set in MIX_LEN
+		before MySQL 5.6.5. The flag should not have been
+		introduced in persistent storage. MariaDB will keep
+		setting the flag when writing SYS_TABLES entries for
+		newly created or rebuilt tables or partitions, but
+		will otherwise ignore the flag. */
 
 		/* Now that we have the proper name for this tablespace,
 		look to see if it is already in the tablespace cache. */

storage/innobase/include/dict0dict.ic

@@ -1534,16 +1534,7 @@ bool
 dict_table_is_file_per_table(
 	const dict_table_t*	table)	/*!< in: table to check */
 {
-	bool is_file_per_table =
-		!is_system_tablespace(table->space);
-
-	/* If the table is file-per-table and it is not redundant, then
-	it should have the flags2 bit for DICT_TF2_USE_FILE_PER_TABLE. */
-	ut_ad(!is_file_per_table
-	      || !DICT_TF_GET_COMPACT(table->flags)
-	      || DICT_TF2_FLAG_IS_SET(table, DICT_TF2_USE_FILE_PER_TABLE));
-
-	return(is_file_per_table);
+	return !is_system_tablespace(table->space);
 }
 
 /** Get reference count.