nir: Add a concept of per-member structs and a lowering pass

This adds a concept of "members" to a variable with an interface type.
It allows you to specify the full variable data for each member of the
interface instead of once for the variable.  We also add a lowering pass
to lower those variables to a sequence of variables and rewrite all the
derefs accordingly.

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>

src/compiler/nir/nir_validate.c

@@ -1100,6 +1100,13 @@ validate_var_decl(nir_variable *var, bool is_global, validate_state *state)
       }
    }
 
+   if (var->num_members > 0) {
+      const struct glsl_type *without_array = glsl_without_array(var->type);
+      validate_assert(state, glsl_type_is_struct(without_array));
+      validate_assert(state, var->num_members == glsl_get_length(without_array));
+      validate_assert(state, var->members != NULL);
+   }
+
    /*
     * TODO validate some things ir_validate.cpp does (requires more GLSL type
     * support)