codegen: add butterfly reduce for compute-3.0 devices
codegen: add butterfly reduce for compute-3.0 devices

Still disabled by default, but gives correct results. Execution configuration needs to be told about it for shared memory / thread block purposes.

mikusp/accelerate-cuda,flowbox-public/accelerate-cuda,tmcdonell/accelerate-cuda,AccelerateHS/accelerate-cuda,tmcdonell/accelerate-cuda,mikusp/accelerate-cuda,mwu-tow/accelerate-cuda,flowbox-public/accelerate-cuda,mwu-tow/accelerate-cuda,flowbox-public/accelerate-cuda,AccelerateHS/accelerate-cuda,tmcdonell/accelerate-cuda,mikusp/accelerate-cuda,AccelerateHS/accelerate-cuda,mwu-tow/accelerate-cuda