[optimize] MRE::typ(): remove call in ReductionPushdown

[mgree]

Part of MirRelationExpr::typ() cleanup.

Skip a call to typ() that is only for arity.

Motivation
This PR adds a known-desirable feature.
https://github.com/MaterializeInc/database-issues/issues/2488
https://github.com/MaterializeInc/database-issues/issues/3779
https://github.com/MaterializeInc/database-issues/issues/4959

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.

[ggevay]

Great!