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.

Tags:
----
REL8_1_STABLE

Modified Files:
--------------
    pgsql/src/backend/optimizer/util:
        var.c (r1.66 -> r1.66.2.1)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/util/var.c?r1=1.66&r2=1.66.2.1)
    pgsql/src/test/regress/expected:
        subselect.out (r1.13.2.3 -> r1.13.2.4)

(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/subselect.out?r1=1.13.2.3&r2=1.13.2.4)
    pgsql/src/test/regress/sql:
        subselect.sql (r1.8.2.3 -> r1.8.2.4)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/subselect.sql?r1=1.8.2.3&r2=1.8.2.4)