[NVVM] Remove noduplicate attribute from synchronizing intrinsics.

Summary:
I've completed my audit of all the code that looks at noduplicate and
added handling of convergent where appropriate, so we no longer need
noduplicate on these intrinsics.

Reviewers: jholewinski

Subscribers: llvm-commits, jholewinski

Differential Revision: http://reviews.llvm.org/D18168

llvm-svn: 264107