[OpenMP] Run rewriteDeviceCodeStateMachine in the Module not CGSCC pass

While rewriteDeviceCodeStateMachine should probably be folded into
buildCustomStateMachine, we at least need the optimization to happen.
This was not reliably the case in the CGSCC pass but in the Module pass
it seems to work reliably.

This also ports a test to the new kernel encoding (target_init/deinit),
and makes sure we cannot run the kernel in SPMD mode.

Differential Revision: https://reviews.llvm.org/D106345