[PATCH RFC 1/1] AMDGPU: Implement get_global_offset builtin

Also fix get_global_id to consider offset
Fixes global-offset piglit and GEGL video-degradation tests on both r600 (Turks) and GCN(Kaveri)

Depends on: http://reviews.llvm.org/D20299

Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>

Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>

Also fix get_global_id to consider offset
No idea how to add this for ptx, so they are stuck with the old get_global_id
implementation.

Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>

LGTM.

LGTM.