Rework coroutine transform to be more flexible in preparation for async generators

[compiler-errors]

My work-in-progress implementation of async gen blocks has a slightly more involved coroutine desugaring, since the generators must return Poll<Option<T>>, instead of just Poll<T> or Option<T>. This means that the state_adt_ref and state_args for the current coroutine transform TransformVisitor are not sufficient.

This PR inlines the TransformVisitor::coroutine_state_adt_and_variant_idx function into its call-sites, simplifies some logic, and reworks TransformVisitor::make_state to be a bit more clear and easy to read.

[rustbot]

r? @cjgillot

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[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[bors]

☔ The latest upstream changes (presumably #118420) made this pull request unmergeable. Please resolve the merge conflicts.

[compiler-errors]

This was subsumed by #118420.