There is certainly some refactoring that should be done in the pipeline frontend -> passes -> backend (including at the level of VSIR), but it would probably be excessive for this MR. I think I can try to work something out of your proposals in the next few days.
I certainly didn't intend to imply that any of that needs to happen in the context of this MR. Just clarifying the direction I'd like to go with vsir_program and vkd3d_shader_parser, since you asked.