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:
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REL8_3_STABLE

Modified Files:
--------------
pgsql/src/backend/optimizer/util:
var.c (r1.73 -> r1.73.2.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/util/var.c?r1=1.73&r2=1.73.2.1)
pgsql/src/test/regress/expected:
subselect.out (r1.16.2.3 -> r1.16.2.4)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/subselect.out?r1=1.16.2.3&r2=1.16.2.4)
pgsql/src/test/regress/sql:
subselect.sql (r1.11.2.3 -> r1.11.2.4)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/subselect.sql?r1=1.11.2.3&r2=1.11.2.4)