gallivm: fix coroutines on aarch64 with llvm 8

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: d32690b43c9 (gallivm: add coroutine pass manager support)
Reviewed-by: Roland Scheidegger <sroland@vmware.com>