[NVPTX] Mark bar.sync intrinsic as convergent.

Summary:
__syncthreads, which corresponds to bar.sync 0, is already convergent.
This makes the more general bar.sync n likewise convergent.

Reviewers: tra

Subscribers: llvm-commits, jholewinski

Differential Revision: http://reviews.llvm.org/D21161

llvm-svn: 272295