pgsql: Fix qual_is_pushdown_safe to not try to push down quals involving

Log Message:
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Fix qual_is_pushdown_safe to not try to push down quals involving a whole-row
Var referencing the subselect output.  While this case could possibly be made
to work, it seems not worth expending effort on.  Per report from Magnus
Naeslund(f).

Tags:
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REL8_1_STABLE

Modified Files:
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    pgsql/src/backend/optimizer/path:
        allpaths.c (r1.137.2.1 -> r1.137.2.2)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/path/allpaths.c.diff?r1=1.137.2.1&r2=1.137.2.2)