Eliminate some typ() calls and an arity() call from MIR lowering

[ggevay]

More typ() call elimination, similar to #30878

Also eliminates an arity call, since the arity can be seen from the type.

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.

[mgree]

Looks good to me!

[mgree]

I think we could do slightly better, sometimes avoiding the arity() call, by doing the following at the top of the function:

let (plan, keys) = self.lower_mir_expr(input)?;
let arity = types.as_ref().map(|types| types.len()).unwrap_or_else(|| input.arity());
let (threshold_plan, required_arrangement) = ThresholdPlan::create_from(arity);