Fix oversight in FindTriggerIncompatibleWithInheritance.

Fix oversight in FindTriggerIncompatibleWithInheritance.

This function is called from ATExecAttachPartition/ATExecAddInherit,
which prevent tables with row-level triggers with transition tables from
becoming partitions or inheritance children, to check if there is such a
trigger on the given table, but failed to check if a found trigger is
row-level, causing the caller functions to needlessly prevent a table
with only a statement-level trigger with transition tables from becoming
a partition or inheritance child.  Repair.

Oversight in commit 501ed02cf.

Author: Etsuro Fujita <etsuro.fujita@gmail.com>
Discussion: https://postgr.es/m/CAPmGK167mXzwzzmJ_0YZ3EZrbwiCxtM1vogH_8drqsE6PtxRYw%40mail.gmail.com
Backpatch-through: 13