sql/stats: support constrained stats using computed columns that can be inferred from predicates

Currently, constrained partial statistics (`CREATE STATISTICS ... WHERE ...`, #93998) require that the `WHERE` clause directly constrain the column being collected on, which must also be the prefix of an index. However, we should also support cases where an index prefix is a computed column that can be inferred from the predicate.

Example:
```
CREATE TABLE t (
x INT NOT NULL,
y INT NOT NULL AS (x % 5) STORED,
PRIMARY KEY (y, x)
);

CREATE STATISTICS stat ON x FROM t WHERE x = 100;
```

Jira issue: CRDB-54328