[NVPTX] Test that MachineSink won't sink across llvm.cuda.syncthreads.
The syncthreads MI is modeled as mayread/maywrite -- convergence doesn't
even come into play here. Nonetheless this property is highly implicit
in the tablegen files, so a test seems appropriate.
Subscribers: llvm-commits, jholewinski
Differential Revision: http://reviews.llvm.org/D17319