The optimizer can generate constrained scans over indexes on computed
columns when columns referenced in the computed column expression are
held constant. Consider this example:

    CREATE TABLE t (a INT, v INT AS (a + 1) STORED, INDEX v_idx (v))
    SELECT * FROM t WHERE a = 1

A constrained scan can be generated over `v_idx` because `v` depends on
`a` and the query filter holds `a` constant.

This commit lifts a restriction that prevented this optimization when
columns referenced in the computed column expression were held constant
to the `NULL` value. As far as I can tell, this restriction is not
necessary. In fact, @rytaft had questioned its purpose originally, but
the question was never answered:

cockroachdb#43450 (review)

By lifting this restriction, the optimizer can explore constrained scans
over both indexed computed columns with `IS NULL` expressions and
expression indexes with `IS NULL` expressions.

Fixes cockroachdb#83390

Release note (performance improvement): The optimizer now explores more
efficient query plans when index computed columns and expressions have
`IS NULL` expressions.