Simplify how coroutine transform and shims handle nested derefs

[cjgillot]

These are a few simplifications I stumbled upon while working on #146562.

The first commit is purely cosmetic.
The second commit handles an early return where deref_finder was not run, ICEing in validate-mir.
The third commit replaces an open-coded version of deref_finder with a call to that method.

[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[rustbot]

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[folkertdev]

r? wg-mir-opt

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.