The prtype & DATA_LONG_TRUE_VARCHAR flag only plays a role when
converting between InnoDB internal format and the MariaDB SQL layer
row format. Ideally this flag would never have been persisted in the
InnoDB data dictionary.

There were bogus assertion failures when an instant ADD, DROP, or
column reordering was combined with a change of extending a VARCHAR
from less than 256 bytes to more than 255 bytes. Such changes are
allowed starting with MDEV-15563 in MariaDB 10.4.3.

dict_table_t::instant_column(), dict_col_t::same_format(): Ignore
the DATA_LONG_TRUE_VARCHAR flag, because it does not affect the
persistent storage format.