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>

src/compiler/spirv/vtn_variables.c

@@ -44,7 +44,7 @@ vtn_access_chain_create(struct vtn_builder *b, unsigned length)
    return chain;
 }
 
-static bool
+bool
 vtn_pointer_uses_ssa_offset(struct vtn_builder *b,
                             struct vtn_pointer *ptr)
 {