[optimizer] Reduce MRE::typ() calls in SemijoinIdempotence

[mgree]

Ensures we only call MRE::typ() twice, rather than six times, in attempt_join_simplification. Changes the interface of distinct_on_keys_of to just take a RelationType, which should offer some additional savings.

Motivation

• This PR refactors existing code.

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.