gallivm: fix coroutines on aarch64 with llvm 8
The coroutine split pass is missing a dependency before LLVM 9.0, and fails to initialise properly if the CallGraphWrapperPass hasn't be initialised earlier (x86 does it due to some of it's passes requiring it).
This is a workaround for llvm 8 (coroutines are only supported in 8 and higher). It adds another pass that has a dependency on the pass the coroutines split requires. This pass shouldn't have any raal effects.
Fixes: d32690b4 (gallivm: add coroutine pass manager support)