Implement async and prefetch built-in.

Implement async and prefetch built-in.

Using the normal load & store to implement async copy,
and so wait_group_events use barrier.
Prefetch just define an empty function.

V2: fix llvm build error.

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