[NVPTX] Mark bar.sync intrinsic as convergent.

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@272295 91177308-0d34-0410-b5e6-96231b3b80d8