Fix a bug in setA(...)
Fix a bug in setA(...)

Previously we use `gpuWorld == globalCpuWorld` to determine whether we
have more CPU ranks than number of GPUs. However, since we use
MPI_split to split communicator no matter regardless of the number of
CPU ranks and GPUs, gpuWorld would always be different from
globalCpuWorld (even their contents are the same). So now we just use
`gpuWorldSize == globalSize` to make the decision.

barbagroup/AmgXWrapper,piyueh/AmgXWrapper