release-22.2: indexrec: fix internal error with inverted indexes

[mgartner]

Backport 1/1 commits from #110116.

/cc @cockroachdb/release

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A bug was introduced in #108576 in code that compares index columns of
an existing index and a hypothetical index. Prior to this commit, this
logic only checked that the hypothetical index column was an inverted
column (the last explicitly indexed column in an inverted index) before
fetching the inverted source column ordinal for both the hypothetical
and existing index column. When the existing index column was not an
inverted column, and internal error would occur.

This commit fixes the bug by checking that both the hypothetical and
existing index columns are inverted columns before fetching the source
column ordinals.

There is no release note because this bug does not exist in any
releases.

Fixes #109974

Release note: None

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Release justification: Fixes internal error in index recommendations.

[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 

[michae2]

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @mgartner)