pgsql: Revise searching of subplan target lists to use something more

Log Message:
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Revise searching of subplan target lists to use something more efficient
than tlist_member calls.  Building a large join tlist is still O(N^2),
but with a much smaller constant factor than before.

Modified Files:
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    pgsql/src/backend/optimizer/plan:
        setrefs.c (r1.110 -> r1.111)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/plan/setrefs.c.diff?r1=1.110&r2=1.111)