[NVPTX] Mark nvvm synchronizing intrinsics as convergent.

Summary:
This is the attribute purpose-made for e.g. __syncthreads.  It appears
that NoDuplicate may not be sufficient to prevent Sink from touching a
call to __syncthreads.

Reviewers: jingyue, hfinkel

Subscribers: llvm-commits, jholewinski, jhen, rnk, tra, majnemer

Differential Revision: http://reviews.llvm.org/D16941

llvm-svn: 260005