release-23.2: opt,plpgsql: subquery hoisting rules should not reorder PL/pgSQL subroutines

[blathers-crl]

Backport 1/1 commits from #120451 on behalf of @DrewKimball.

/cc @cockroachdb/release

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Due to #97432, it is possible for subquery-hoisting decorrelation rules to hoist a volatile subquery from a CASE expression. This can cause a query to display side effects which were meant to be gated behind a conditional expression, or else were meant to occur in a different order. This is a problem for PL/pgSQL, which relies on expressions being executed in a certain order. While #115826 added a Barrier expression to prevent rules from changing execution order, this doesn't work for hoisting rules that traverse an entire operator subtree, instead of relying on match-and-replace patterns.

This commit makes a targeted fix for PL/pgSQL routines by preventing subquery-hoisting rules from matching if a scalar expression contains a BarrierExpr or a UDFCall with TailCall = true. Either of these conditions indicates that changing execution order would cause incorrect results.

Fixes #120439

Release note (bug fix): Fixed a bug introduced in v23.2 that could cause a PL/pgSQL routine to return incorrect results when there was at least one parameter, and an IF statement with one leak-proof branch, and one branch with side effects.

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Release justification: targeted fix for PL/pgSQL routines

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

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[cockroach-teamcity]

This change is 

[github-actions]

Reminder: it has been 3 weeks please merge or close your backport!

[mgartner]

Reviewed 3 of 3 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @DrewKimball, @rafiss, and @yuzefovich)

[mgartner]

Friendly ping @DrewKimball - we good to merge?

[DrewKimball]

TFYR