Rework mz_dataflow_operator_reachability_per_worker join condition to be more clearly an equivalence

[frankmcsherry]

Our built in view for mz_dataflow_operator_reachability_per_worker had a join condition in a CASE statement. By pulling the equivalence out of the case statement, and making it equate a value that is a CASE statement, the join planner can recognize it as an equijoin.

As a result, queries to mz_dataflow_operator_reachability_per_worker and derived views take substantially less time (from minutes to seconds).

Issue https://github.com/MaterializeInc/database-issues/issues/9055 describes automating this transformation.

Motivation

Tips for reviewer

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.

[antiguru]

Thank you!