pgsql: Fix an oversight in convert_EXISTS_sublink_to_join: we can't

Log Message:
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Fix an oversight in convert_EXISTS_sublink_to_join: we can't convert an
EXISTS that contains a WITH clause.  This would usually lead to a
"could not find CTE" error later in planning, because the WITH wouldn't
get processed at all.  Noted while playing with an example from Ken Marshall.

Modified Files:
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    pgsql/src/backend/optimizer/plan:
        subselect.c (r1.157 -> r1.158)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/plan/subselect.c?r1=1.157&r2=1.158)