This is required by @nikic in https://reviews.llvm.org/D127383 and this fixes a FIXME too.
@rjmccall I'm not sure if swift have emitted "coroutine.presplit" in the frontend. If it haven't do so, it should be fine.
@ezhulenev I'm not sure if the changes in mlir part calls the right API. Although it passes the tests, I haven't run mlir actually.