Check lateral references within PHVs for memoize cache keys

If we intend to generate a memoize node atop a path, we need some kind
of cache key. Currently we search the path's parameterized clauses and
its parent's lateral_vars for that. ISTM this is not sufficient because
their might be lateral references derived from PlaceHolderVars, which
can also act as cache key but we neglect to take into consideration. As
an example, consider

create table t(a int);
insert into t values (1), (1), (1), (1);
analyze t;

explain (costs off) select * from t t1 left join lateral (select t1.a as
t1a, t2.a as t2a from t t2) s on true where s.t1a = s.t2a;
QUERY PLAN
----------------------------
Nested Loop
-> Seq Scan on t t1
-> Seq Scan on t t2
Filter: (t1.a = a)
(4 rows)

We cannot find available cache keys for memoize node because the inner
side has neither parameterized path clauses nor lateral_vars. However
if we are able to look in the PHV for lateral references, we will find
the cache key 't1.a'.

Actually we do have checked PHVs for lateral references, earlier in
create_lateral_join_info. But that time we only marked lateral_relids
and direct_lateral_relids, without remembering the lateral expressions.
So I'm wondering whether we can fix that by fetching Vars (or PHVs) of
lateral references within PlaceHolderVars and remembering them in the
baserel's lateral_vars.

Attach a draft patch to show my thoughts.

Thanks
Richard