[repr types] eliminate noop casts (transform impl)

[mgree]

The varchar_to_text cast is functionally a noop, but exists to satisfy the typechecker: varchar(8) and varchar and text are different SQL types.

But they are not different representation types---all of these will be Datum::String at runtime. So: in MIR, we can eliminate this cast.

This PR introduces a new transform (gated behind the feature flag enable_cast_elimination) that removes varchar_to_text first thing in optimization (in the fast-path optimizer, the logical_optimizer, and manually in one test case).

#34350 is an alternative (and, I think, better) implementation that simply avoids generating varchar_to_text during lowering.

Both PRs will need to have only the repr typechecker running to avoid inconsistencies #34351.

Motivation

• This PR adds a known-desirable feature.
  design doc: MIR typechecking using representation types #27239

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.