Change DeviceCount() to use the new host/device code pattern
Change DeviceCount() to use the new host/device code pattern

The function DeviceCount was still using "#if CUB_PTX_ARCH == 0" to separate
host code and device code.  Change it to use the new pattern of
"if (CUB_IS_HOST_CODE)".

NVlabs/CUB,NVlabs/cub,NVIDIA/cub