Sufficient for compiling a no-op pixel shader.
This should probably be rebased on top of !263 because it introduces vkd3d_spirv_get_type_id_for_data_type(), which 263 renders unnecessary.
!263 is not essential, but I think using two different type systems in the backend is not ideal.
-- v3: vkd3d-shader/dxil: Emit undefined constants. vkd3d-shader/spirv: Introduce an undefined register type. vkd3d-shader/dxil: Emit the shader instructions. vkd3d-shader/spirv: Do not normalise Shader Model 6 shaders.