release-21.2: opt: fix missing filters after join reordering

[mgartner]

Backport 1/2 commits from #76334.

/cc @cockroachdb/release

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opt: fix missing filters after join reordering

This commit eliminates logic in the assoc, leftAsscom, and
rightAsscom functions in the join order builder that aimed to prevent
generating "orphaned" predicates, where one or more referenced relations
are not in a join's input. In rare cases, this logic had the side effect
of creating invalid conflict rules for edges, which could prevent valid
predicates from being added to reordered join trees.

It is safe to remove these conditionals because they are unnecessary.
The CD-C algorithm already prevents generation of orphaned predicates by
checking that the total eligibility set (TES) is a subset of a join's
input vertices. In our implementation, this is handled by the
checkNonInnerJoin and checkInnerJoin functions.

Fixes #76522

Release note (bug fix): A bug has been fixed which caused the query optimizer
to omit join filters in rare cases when reordering joins, which could
result in incorrect query results. This bug was present since v20.2.

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Release justification: This fixes a logical correctness bug in
the optimizer's join reordering logic.

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Patches should only be created for serious issues or test-only changes.
• Patches should not break backwards-compatibility.
• Patches should change as little code as possible.
• Patches should not change on-disk formats or node communication protocols.
• Patches should not add new functionality.
• Patches must not add, edit, or otherwise modify cluster versions; or add version gates.

If some of the basic criteria cannot be satisfied, ensure that the exceptional criteria are satisfied within.

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters.
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.

Add a brief release justification to the body of your PR to justify this backport.

Some other things to consider:

• What did we do to ensure that a user that doesn’t know & care about this backport, has no idea that it happened?
• Will this work in a cluster of mixed patch versions? Did we test that?
• If a user upgrades a patch version, uses this feature, and then downgrades, what happens?

[cockroach-teamcity]

This change is 

[rytaft]

Reviewed 5 of 5 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @msirek)