[CIR] Remove branch through cleanup fixups (#182953)
Because we are using a structured representation of cleanups in CIR, we
don't need to handle branching through cleanups during codegen. These
branches are created during CFG flattening instead. However, we had
already committed some code that copied the classic codegen behavior for
branching through cleanups. This change deletes that unneeded code.
The most significant change here is that when we encounter a return
statement we emit the return directly in the current location.
The coroutine implementation still creates a return block in the current
lexical scope and branches to that block. Cleaning up that
representation is left as future work.
The popCleanupBlock handling still has a significant amount of logic
that is carried over from the classic codegen implementation. It is left
in place until we can be sure we won't need it.
