[NVPTX] Annotate convergent intrinsics as convergent.

Summary:
Previously the machine instructions for bar.sync &co. were not marked as
convergent.  This resulted in some MI passes (such as TailDuplication,
fixed in an upcoming patch) doing unsafe things to these instructions.

Reviewers: jingyue

Subscribers: llvm-commits, tra, jholewinski, hfinkel

Differential Revision: http://reviews.llvm.org/D17318

llvm-svn: 261115