pgsql: Calculation of keys_are_unique flag was wrong for cases involving

Log Message:
-----------
Calculation of keys_are_unique flag was wrong for cases involving
redundant cross-datatype comparisons. Per example from Merlin Moncure.

Modified Files:
--------------
pgsql/src/backend/access/nbtree:
nbtutils.c (r1.60 -> r1.61)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/nbtutils.c.diff?r1=1.60&r2=1.61)