nir: Add SSBO->global lowering pass

nir: Add SSBO->global lowering pass

To facilitate lowering SSBOs to globals, we need a load_ssbo_address
intrinsic. This intrinsic takes an SSBO index and loads the address in
global memory of the SSBO (likely implemented via a uniform in the
driver). In the future, we'll support bounds checking, but at the moment
this is not supported (this pass should only be used for trusted
contexts at the moment, i.e. contexts without robustness extensions).

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/2753>