We must relax too strict debug assertions. For latin1_swedish_ci,
mtype=DATA_CHAR or mtype=DATA_VARCHAR will be used instead of
mtype=DATA_MYSQL or mtype=DATA_VARMYSQL. Likewise, some changes of
dtype_get_charset_coll() do not affect the data type encoding,
but only any indexes that are defined on the column.

Charset::same_encoding(): Check whether two charset-collations have
the same character set encoding.

dict_col_t::same_encoding(): Check whether two character columns
have the same character set encoding.

dict_col_t::same_type(): Check whether two columns have a compatible
data type encoding.

dict_col_t::same_format(), dict_table_t::instant_column(): Do not
compare mtype or the charset-collation of prtype directly.
Rely on dict_col_t::same_type() instead.

dtype_get_charset_coll(): Narrow the return type to uint16_t.

This is a refined version of a fix that was developed by
Thirunarayanan Balathandayuthapani.