Previously, the SimplifyPartialIndexProjections rule incorrectly
simplified partial index PUT and DEL columns that were reused for
multiple partial indexes. This caused index corruption which could
result in incorrect query results.

For example, consider:

    CREATE TABLE t (
      a INT,
      b INT,
      c INT,
      INDEX a_idx (a) WHERE c IS NULL,
      INDEX b_idx (b) WHERE c IS NULL
    )

    UPDATE t SET a = NULL

In the UPDATE, a single column will be synthesized for the PUT and DEL
columns of both partial indexes. The UPDATE does not mutate columns in
b_idx, so the synthesized column was simplified to false. However, this
caused index corruption of a_idx because a_idx contains mutating columns
and it may require writes.

The rule has been updated so that synthesized PUT and DEL columns are
only simplified to false if they are used for a single partial index.

Fixes cockroachdb#74385

Release note (bug fix): A bug has been fixed which caused corruption of
partial indexes, which could cause incorrect query results. The bug was
only present when two or more partial indexes in the same table had
identical `WHERE` clauses. This bug has been present since version
21.1.0.