indexrec: fix internal error with inverted indexes

[mgartner]

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

[cockroach-teamcity]

This change is 

[DrewKimball]

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @michae2)

[michae2]

Gah, good catch!

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

[mgartner]

TFTRs!

bors r+

[craig]

Build succeeded:

• Bazel Essential CI (Cockroach)

[blathers-crl]

Encountered an error creating backports. Some common things that can go wrong:

1. The backport branch might have already existed.
2. There was a merge conflict.
3. The backport branch contained merge commits.

You might need to create your backport manually using the backport tool.

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error creating merge commit from b10ee46 to blathers/backport-release-22.2-110116: POST https://api.github.com/repos/cockroachdb/cockroach/merges: 409 Merge conflict []

you may need to manually resolve merge conflicts with the backport tool.

Backport to branch 22.2.x failed. See errors above.

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