microsoft/compiler: use nir_shader_add_variable

Now that we're using the right variable-mode, this is the same, and
prepares us a bit more for the future where all variables will be in the
same list.

src/microsoft/compiler/nir_to_dxil.c

@@ -4263,7 +4263,7 @@ append_input_or_sysvalue(struct ntd_context *ctx, nir_shader *s,
    if (!ctx->system_value[sv_slot])
       return false;
 
-   exec_list_push_tail(&s->system_values, &ctx->system_value[sv_slot]->node);
+   nir_shader_add_variable(s, ctx->system_value[sv_slot]);
    return true;
 }