Bug #121646 Wrong result: EXISTS predicate lost in semijoin materialization inside nested outer joins

Description:
MySQL Community Server 9.7.1 returns incorrect results after the `replace_where_with_true` mutation relaxes the outer WHERE clause of a SELECT containing nested outer joins. The original returns 5,184 NULL values, but the mutated query returns 5,184 copies of `1`. Relaxing WHERE must retain every original result row, including multiplicities; the observed results violate that requirement.

The reduced reproduction uses two single-column tables and 73 input rows. The only query change is `WHERE a.c IS NOT NULL` to `WHERE 1 = 1`, emitted by the project's existing mutation operator. Both queries use default optimizer settings without hints.

The mutated execution plan uses semijoin materialization and omits the false subquery predicate `s.c > 1`. The original uses FirstMatch and retains that filter. These observations identify a failing optimization path; the precise internal cause has not been established.

## Expected behaviour

Both queries: 5,184 rows, each containing NULL.

## Actual behaviour

| Query | Rows | Value |
|---|---|---|
| Original | 5,184 | NULL |
| Mutated | 5,184 | `1` |

How to repeat:
-- Run in a fresh test database.
SET SESSION optimizer_switch = DEFAULT;

CREATE TABLE valscope_exists_outer (c BLOB);
CREATE TABLE valscope_exists_inner (c INT);

INSERT INTO valscope_exists_outer (c)
WITH RECURSIVE seq(n) AS (
    SELECT 1
    UNION ALL
    SELECT n + 1 FROM seq WHERE n < 72
)
SELECT 'x' FROM seq;
INSERT INTO valscope_exists_inner VALUES (1);

ANALYZE TABLE valscope_exists_outer, valscope_exists_inner;

-- Original: expected and actual 5,184 NULL values.
SELECT b.c
FROM valscope_exists_outer AS a
LEFT JOIN valscope_exists_inner AS b
    ON EXISTS (
        SELECT 1
        FROM valscope_exists_inner AS s
        WHERE s.c > 1
    )
RIGHT JOIN valscope_exists_outer AS d
    ON a.c IS NOT NULL
WHERE a.c IS NOT NULL;

-- Mutated by replace_where_with_true: expected 5,184 NULL values,
-- actual 5,184 copies of 1.
SELECT b.c
FROM valscope_exists_outer AS a
LEFT JOIN valscope_exists_inner AS b
    ON EXISTS (
        SELECT 1
        FROM valscope_exists_inner AS s
        WHERE s.c > 1
    )
RIGHT JOIN valscope_exists_outer AS d
    ON a.c IS NOT NULL
WHERE 1 = 1;

Hi cl hl,

Thank you for the test case. Verified as described. However, we think this is a duplicate of Bug#113718. So we are closing this report.