pgsql: Teach predtest.c how to reason about ScalarArrayOpExpr clauses as

Log Message:
-----------
Teach predtest.c how to reason about ScalarArrayOpExpr clauses as though
they were broken-out AND or OR lists. The least grotty way to do this
seemed to be to set up a general mechanism for handling nodes as though
they were ANDs or ORs. There's no other immediate use for it, but perhaps
we might want to use the mechanism someday for things like BETWEEN
SYMMETRIC.

Modified Files:
--------------
pgsql/src/backend/optimizer/util:
predtest.c (r1.4 -> r1.5)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/util/predtest.c.diff?r1=1.4&r2=1.5)