radv: handle loading from shared pointers

radv: handle loading from shared pointers

We won't have a var to load from, so don't try to the processing
required if we don't need it.

This avoids crashes in:
dEQP-VK.spirv_assembly.instruction.compute.variable_pointers.compute.workgroup_two_buffers

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>