pgsql: Teach simplify_boolean_equality to simplify the forms foo <> true

Log Message:
-----------
Teach simplify_boolean_equality to simplify the forms foo <> true and
foo <> false, along with its previous duties of simplifying foo = true
and foo = false.  (All of these are equivalent to just foo or NOT foo
as the case may be.)  It's not clear how often this is really useful;
but it costs almost nothing to do, and it seems some people think we
should be smart about such cases.  Per recent bug report.

Modified Files:
--------------
    pgsql/src/backend/optimizer/util:
        clauses.c (r1.277 -> r1.278)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/util/clauses.c?r1=1.277&r2=1.278)
    pgsql/src/include/catalog:
        pg_operator.h (r1.166 -> r1.167)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_operator.h?r1=1.166&r2=1.167)