opt: constrain expression indexes with IS NULL expressions

[mgartner]

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:

#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 #83390

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

[cockroach-teamcity]

This change is 

[msirek]

Reviewed 3 of 3 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @rytaft)

[mgartner]

@rytaft do you mind taking a look? Can you think of any reasons why the check I've removed is required?

[mgartner]

@rytaft friendly ping :)

[rytaft]

Thanks! Sorry for the delay!

Reviewed 3 of 3 files at r1, all commit messages.
Reviewable status: complete! 2 of 0 LGTMs obtained (waiting on @mgartner)

[mgartner]

TFTR!

bors r+

[craig]

Build succeeded:

• GitHub CI (Cockroach)