nir: Add load/store/atomic global intrinsics

nir: Add load/store/atomic global intrinsics

These correspond roughly to reading/writing OpenCL global pointers.  The
idea is that they just take a bare address and load/store from it.  Of
course, exactly what this address means is driver-dependent.

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Karol Herbst <kherbst@redhat.com>