nir: Add load_ssbo_address/size intrinsics
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). Further, to support bounds checking during lowering, we have a load_ssbo_size intrinsic used as the upper bound on offsets (again, likely implemented with a uniform).
Signed-off-by: Alyssa Rosenzweig email@example.com