In #9211 we incorporated more one-off form-fitting fixes for block arguments processing. Two separate changes were necessary to support both interpreter and JIT. We need to refactor, clean up, and unify block argument processing better.

This will require evaluating the existing logic paths and ensuring we understand all argument forms (call arguments and parameter layout) and take each path in turn and ensure it is represented clearly in the logic.

This will also help future efforts to pass block arguments straight through, without the boxing currently required to support limited call paths in Block and BlockBody.