Fix the doubly questional fix for MySQL Bug#17250787:
* it detected autoinc index by looking for the first index
that starts from autoinc column. never mind one column
can be part of many indexes.
* it used autoinc_field->field_index to look up into internal
innodb dictionary. But field_index accounts for virtual
columns too, while innodb dictionary ignores them.
Find the index by its name, like elsewhere in ha_innobase.
