spirv: Choose atomic deref type with pointer_uses_ssa_offset

spirv: Choose atomic deref type with pointer_uses_ssa_offset

Previously, we hard-coded the rule about workgroup variables and the
builder lower_workgroup_access_to_offsets flag.  Instead base it on the
handy helper we have for exactly this sort of thing.

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>