nir/lower_system_values: Assert/assume direct var derefs

nir/lower_system_values: Assert/assume direct var derefs

System values are never arrays or structs so we can assume a direct var
deref.  This simplifies things a bit and prevents us from accidentally
throwing away an array index.

Suggested-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Acked-by: Rob Clark <robdclark@gmail.com>
Acked-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Acked-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>