Ideally, we would move some code from
ha_innobase::prepare_inplace_alter_table() to
ha_innobase::check_if_supported_inplace_alter(),
but the API does not really allow us to return errors; it can
only inform which forms of ALGORITHM and LOCK are allowed.

So, we have to duplicate some logic between the "check" and "prepare"
phases. We do the duplication by calling common functions.

instant_alter_column_possible(): Check if instant column operation
is possible. Invoked from both
ha_innobase::check_if_supported_inplace_alter() and
prepare_inplace_alter_table_dict().

ha_innobase::check_if_supported_inplace_alter(): Before refusing
certain operations if FULLTEXT INDEX exist, check if instant ALTER TABLE
is possible and return early if it is the case.

prepare_inplace_alter_table_dict(): Before checking the limitations
on FULLTEXT INDEX, check if instant ALTER TABLE is possible, and suppress
the checks if it is the case. If instant ADD COLUMN is used when the
table already contains FULLTEXT INDEX, do account for a
hidden FTS_DOC_ID_INDEX in a debug assertion.