cudev: Add __shfl_down implementation for long long and unsigned long on devices of CC < 7.0

[cudawarped]

Draft fix for #3962.

Support for __shfl_down on long long was not introduced until CUDA Toolkit 9.0. I don't know if this is just software support or if hardware support was added as well. Its a long shot but it may be the reason that the tests are failing on Compute Capbability 5.3 devices.

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