Fix oversight in construction of sort/unique plans for UniquePaths. If the original IN operator is cross-type, for example int8 = int4, we need to use int4 < int4 to sort the inner data and int4 = int4 to unique-ify it.  We got the first part of that right, but tried to use the original IN operator for the equality checks.  Per bug #5472 from Vlad Romascanu.
Fix oversight in construction of sort/unique plans for UniquePaths.
If the original IN operator is cross-type, for example int8 = int4,
we need to use int4 < int4 to sort the inner data and int4 = int4
to unique-ify it.  We got the first part of that right, but tried to
use the original IN operator for the equality checks.  Per bug #5472
from Vlad Romascanu.

Backpatch to 8.4, where the bug was introduced by the patch that unified
SortClause and GroupClause.  I was able to take out a whole lot of on-the-fly
calls of get_equality_op_for_ordering_op(), but failed to realize that
I needed to put one back in right here :-(

lisakowen/gpdb,arcivanov/postgres-xl,pavanvd/postgres-xl,greenplum-db/gpdb,50wu/gpdb,lisakowen/gpdb,xinzweb/gpdb,pavanvd/postgres-xl,tpostgres-projects/tPostgres,greenplum-db/gpdb,ashwinstar/gpdb,snaga/postgres-xl,pavanvd/postgres-xl,postmind-net/postgres-xl,ovr/postgres-xl,Postgres-XL/Postgres-XL,jmcatamney/gpdb,jmcatamney/gpdb,50wu/gpdb,yazun/postgres-xl,yazun/postgres-xl,lisakowen/gpdb,50wu/gpdb,xinzweb/gpdb,zeroae/postgres-xl,pavanvd/postgres-xl,ovr/postgres-xl,ovr/postgres-xl,Postgres-XL/Postgres-XL,snaga/postgres-xl,ashwinstar/gpdb,Postgres-XL/Postgres-XL,zeroae/postgres-xl,oberstet/postgres-xl,xinzweb/gpdb,tpostgres-projects/tPostgres,ashwinstar/gpdb,ashwinstar/gpdb,ashwinstar/gpdb,postmind-net/postgres-xl,oberstet/postgres-xl,arcivanov/postgres-xl,kmjungersen/PostgresXL,postmind-net/postgres-xl,snaga/postgres-xl,greenplum-db/gpdb,adam8157/gpdb,50wu/gpdb,Postgres-XL/Postgres-XL,zeroae/postgres-xl,yazun/postgres-xl,lisakowen/gpdb,ashwinstar/gpdb,arcivanov/postgres-xl,zeroae/postgres-xl,adam8157/gpdb,kmjungersen/PostgresXL,postmind-net/postgres-xl,xinzweb/gpdb,lisakowen/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,oberstet/postgres-xl,xinzweb/gpdb,snaga/postgres-xl,arcivanov/postgres-xl,xinzweb/gpdb,lisakowen/gpdb,greenplum-db/gpdb,kmjungersen/PostgresXL,tpostgres-projects/tPostgres,adam8157/gpdb,arcivanov/postgres-xl,greenplum-db/gpdb,techdragon/Postgres-XL,jmcatamney/gpdb,adam8157/gpdb,jmcatamney/gpdb,techdragon/Postgres-XL,arcivanov/postgres-xl,yazun/postgres-xl,lisakowen/gpdb,techdragon/Postgres-XL,snaga/postgres-xl,adam8157/gpdb,kmjungersen/PostgresXL,jmcatamney/gpdb,xinzweb/gpdb,zeroae/postgres-xl,Postgres-XL/Postgres-XL,ashwinstar/gpdb,tpostgres-projects/tPostgres,tpostgres-projects/tPostgres,jmcatamney/gpdb,techdragon/Postgres-XL,greenplum-db/gpdb,oberstet/postgres-xl,pavanvd/postgres-xl,lisakowen/gpdb,50wu/gpdb,adam8157/gpdb,adam8157/gpdb,jmcatamney/gpdb,xinzweb/gpdb,ovr/postgres-xl,50wu/gpdb,greenplum-db/gpdb,oberstet/postgres-xl,adam8157/gpdb,kmjungersen/PostgresXL,yazun/postgres-xl,ashwinstar/gpdb,50wu/gpdb,50wu/gpdb,postmind-net/postgres-xl,ovr/postgres-xl,techdragon/Postgres-XL