pgsql: Tweak trivial_subqueryscan() to consider a SubqueryScan's

Log Message:
-----------
Tweak trivial_subqueryscan() to consider a SubqueryScan's targetlist
trivial if it contains either Vars referencing the corresponding subplan
columns, or Consts equaling the corresponding subplan columns.  This
lets the planner eliminate the SubqueryScan in some cases generated by
generate_setop_tlist().

Modified Files:
--------------
    pgsql/src/backend/optimizer/plan:
        setrefs.c (r1.124 -> r1.125)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/plan/setrefs.c.diff?r1=1.124&r2=1.125)