Don't mark partitioned indexes invalid unnecessarily

Don't mark partitioned indexes invalid unnecessarily

When an indexes is created on a partitioned table using ONLY (don't
recurse to partitions), it gets marked invalid until index partitions
are attached for each table partition.  But there's no reason to do this
if there are no partitions ... and moreover, there's no way to get the
index to become valid afterwards, because all partitions that get
created/attached get their own index partition already attached to the
parent index, so there's no chance to do ALTER INDEX ... ATTACH PARTITION
that would make the parent index valid.

Fix by not marking the index as invalid to begin with.

This is very similar to 9139aa19423b, but the pg_dump aspect does not
appear to be relevant until we add FKs that can point to PKs on
partitioned tables.  (I tried to cause the pg_upgrade test to break by
leaving some of these bogus tables around, but wasn't able to.)

Making this change means that an index that was supposed to be invalid
in the insert_conflict regression test is no longer invalid; reorder the
DDL so that the test continues to verify the behavior we want it to.

Author: Álvaro Herrera
Reviewed-by: Amit Langote
Discussion: https://postgr.es/m/20181203225019.2vvdef2ybnkxt364@alvherre.pgsql