Like all IF NOT EXISTS syntax, a Note should be generated.

The original commit of Seqeuences cleared the IF NOT EXISTS part
in the sql/sql_yacc.yy with lex->create_info.init(). Without this
bit set there was no way it could do anything other than error.

To remedy this removal, the sql_yacc.yy components have been
minimised as they where all set at the beginning of the ALTER.
This way the opt_if_not_exists correctly set the IF_EXISTS flag.

In MDEV-13005 (bb4dd70e) the error code changed, requiring
ER_UNKNOWN_SEQUENCES to be handled in the function
No_such_table_error_handler::handle_condition.