The host registration is a convenient way to get CUDA kernels
running, but it may be slow and does not work for all buffer
(like global constants). This revision uses the proper alloc
copy dealloc chains for buffers, using asynchronous chains
to increase overlap. The host registration mechanism is
kept under a flag for the output, just for experimentation
purposes while this project ramps up.
Reviewed By: Peiming
Differential Revision: https://reviews.llvm.org/D148682
86888e420c