trx_purge_add_undo_to_history() is not optimal

The function trx_purge_add_undo_to_history(), which is called after each transaction commit or rollback that had modified persistent tables, is not optimal. It calls trx_undo_set_state_at_finish() in a different compilation unit and evaluates a condition on undo->state several times. We could get a small performance improvement by simplifying this.