release-25.2: plpgsql: move barrier after outer-join for SQL stmt with INTO clause

[blathers-crl]

Backport 1/1 commits from #156966 on behalf of @DrewKimball.

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PL/pgSQL routines use barrier expressions to prevent optimizations that would remove side effecting expressions from the query plan. Previously, we would add a barrier below the outer join used to ensure at least one row returned by a SQL statement with an INTO clause. However, this wasn't enough, since after some inlining and projection push-down into the join, the join could be eliminated. This could result in a side-effecting SQL statement being dropped if its target variable(s) weren't referenced.

This commit fixes the bug by moving the barrier above the outer-join. This prevents optimizations (like join elimination) that rely on proving that the columns from the null-extended side of the join are not needed.

Fixes #147269

Release note (bug fix): Fixed a bug existing since SQL statements with INTO clause were introduced for PL/pgSQL routines in v23.2. The bug could cause a SQL statement with side effects (e.g. INSERT) to be dropped if none of the target variables from the INTO clause were referenced.

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Release justification:

[blathers-crl]

Thanks for opening a backport.

Before merging, please confirm that it falls into one of the following categories (select one):

• Non-production code changes. Includes test-only changes, build system changes, etc.
• Fixes for serious issues. Defined in the policy as correctness, stability, or security issues, data corruption/loss, significant performance regressions, breaking working and widely used functionality, or an inability to detect and debug production issues.
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[cockroach-teamcity]

This change is 

[michae2]

@michae2 reviewed 4 of 4 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @mgartner, @mw5h, and @yuzefovich)