pgsql: Fix "cannot handle unplanned sub-select" error that can occur

Log Message:
-----------
Fix "cannot handle unplanned sub-select" error that can occur when a
sub-select contains a join alias reference that expands into an expression
containing another sub-select.  Per yesterday's report from Merlin Moncure
and subsequent off-list investigation.

Back-patch to 7.4.  Older versions didn't attempt to flatten sub-selects in
ways that would trigger this problem.

Modified Files:
--------------
    pgsql/src/backend/optimizer/util:
        var.c (r1.87 -> r1.88)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/util/var.c?r1=1.87&r2=1.88)
    pgsql/src/test/regress/expected:
        subselect.out (r1.23 -> r1.24)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/subselect.out?r1=1.23&r2=1.24)
    pgsql/src/test/regress/sql:
        subselect.sql (r1.15 -> r1.16)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/subselect.sql?r1=1.15&r2=1.16)