Semantically redundant OR FALSE prevents IN-subquery pull-up and causes a slower SubPlan

## Description

This issue concerns the Boolean identity `P OR FALSE = P`. The equivalence
remains valid under SQL three-valued logic, including when `P` evaluates to
NULL. When `P` is an `IN` subquery, PostgreSQL nevertheless optimizes the
two forms differently.

### Expected Behaviour

PostgreSQL should simplify `P OR FALSE` early enough that both forms are
planned identically. In particular, the redundant constant should not
prevent an `IN` subquery from being pulled up into a semijoin.

### Actual Behaviour

The plain `IN` predicate becomes a `Hash Semi Join`. Wrapping the same
predicate in `OR FALSE` leaves it as a hashed `SubPlan` attached to an outer
sequential scan. In the standalone case, execution time increases from 7.143
ms to 13.810 ms, approximately 1.93x.

The generated pair 837 shows a much larger order-stable instance: median
time increases from 2.750 ms to 668.339 ms, approximately 243.9x.

## How to repeat

```sql
DROP TABLE IF EXISTS identity_outer;
DROP TABLE IF EXISTS identity_inner;

CREATE TABLE identity_outer (v INTEGER NOT NULL);
CREATE TABLE identity_inner (v INTEGER NOT NULL);

INSERT INTO identity_outer
SELECT g FROM generate_series(90001, 91000) AS g;

INSERT INTO identity_inner
SELECT g FROM generate_series(1, 100000) AS g;

ANALYZE identity_outer;
ANALYZE identity_inner;

-- Original form P.
EXPLAIN (ANALYZE, BUFFERS, VERBOSE, SETTINGS, TIMING OFF)
SELECT COUNT(*)
FROM identity_outer AS o
WHERE o.v IN (
   SELECT i.v FROM identity_inner AS i
);

-- Equivalent form P OR FALSE.
EXPLAIN (ANALYZE, BUFFERS, VERBOSE, SETTINGS, TIMING OFF)
SELECT COUNT(*)
FROM identity_outer AS o
WHERE (
   o.v IN (SELECT i.v FROM identity_inner AS i)
) OR FALSE;
```

Both queries return 1,000. Characteristic plans and measured times are:

```text
P:
 Hash Semi Join
 Execution Time: 7.143 ms

P OR FALSE:
 Seq Scan on identity_outer
   Filter: ANY (... hashed SubPlan 1 ...)
 Execution Time: 13.810 ms
```