Fix a bug that index restriction by name doesn't work for UPDATEs on

Fix a bug that index restriction by name doesn't work for UPDATEs on
inheritance parent.

Inheritance planner doesn't claim for the inheritance parent relation,
so pg_hint_plan_get_relation_info cannot have the chance to prepare
the index spec information for chlid relations. This change make the
function to try to find parent relation even if it is called for the
children with inhparent == false from the first.