Unprocessed SubLink from whole-row join alias expansion

Further fuzzing with Claude on the join alias found this bug. The
following queries fail in various ways on master and all supported
branches:

create table t (a int, b int);

select 1 from ((select (select 1) as x) s cross join t) j
where (select 1 where j is null) is null;
ERROR: cannot handle unplanned sub-select

select 1 from ((select (select 1) as x) s cross join t) j
where (1, 1) in (select (j is null)::int, count(*) from t);
TRAP: failed Assert("!IsA(node, SubLink)"), File: "prepagg.c"

select 1 from ((select (select 1) as x) s cross join t) j
where exists (select 1 from t tablesample system ((j is null)::int * 100));
ERROR: unrecognized node type: 22

Once subquery s is flattened, the joinaliasvars entry for j.x is no
longer a Var but the SubLink (select 1), so expanding a reference to j
inside a sub-select inserts a SubLink into that sub-select.

But flatten_join_alias_vars_mutator fails to notice this and thus does
not set the sub-select's hasSubLinks flag. So preprocess_expression
skips SS_process_sublinks, and the SubLink survives into code that
can't cope with one.

The fix is to make the same checkExprHasSubLink() test in the
whole-row path. See attached.

- Richard