[NVPTX] Test that MachineSink won't sink across llvm.cuda.syncthreads.
[NVPTX] Test that MachineSink won't sink across llvm.cuda.syncthreads.

Summary:
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.

Reviewers: jingyue

Subscribers: llvm-commits, jholewinski

Differential Revision: http://reviews.llvm.org/D17319

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@261114 91177308-0d34-0410-b5e6-96231b3b80d8

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