[CUDA] Implement atomicInc and atomicDec builtins

These functions cannot be implemented as atomicrmw or cmpxchg
instructions, so they are implemented as a call to the NVVM intrinsics
@llvm.nvvm.atomic.load.inc.32.p0i32 and
@llvm.nvvm.atomic.load.dec.32.p0i32.

Patch by Jason Henline.

Reviewers: jlebar

Differential Revision: http://reviews.llvm.org/D18322

llvm-svn: 264009