pgsql: Allow planner to fold "stable" functions to constants when

Log Message:
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Allow planner to fold "stable" functions to constants when forming
selectivity estimates, per recent discussion.

Modified Files:
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pgsql/src/backend/optimizer/util:
clauses.c (r1.183 -> r1.184)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/util/clauses.c.diff?r1=1.183&r2=1.184)