nir/spirv: Rework function argument setup

Now that we have proper pointer types, we can be more sensible about the
way we set up function arguments and deal with the two cases of pointer
vs. SSA parameters distinctly.

Reviewed-by: Connor Abbott <cwabbott0@gmail.com>

src/compiler/spirv/vtn_variables.c

@@ -233,6 +233,8 @@ vtn_pointer_for_variable(struct vtn_builder *b,
 
    pointer->mode = var->mode;
    pointer->type = var->type;
+   assert(ptr_type->base_type == vtn_base_type_pointer);
+   assert(ptr_type->deref->type == var->type->type);
    pointer->ptr_type = ptr_type;
    pointer->var = var;