Add atomic help functions.

Add atomic help functions.

Use the data port message "Untyped Atomic Operation" to implement openCL atomic functions.
This message can handle both global memory and SLM.
For all atomic functions, the operands will be treated as unsigned int, except imax/imin.
Only add one opcode Atomic for all atomic functions in ir, and identify the atomic function
by ir::AtomicOps.

Signed-off-by: Yang Rong <rong.r.yang@intel.com>
Reviewed-by: Zhigang Gong <zhigang.gong@linux.intel.com>