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.