Normalise the incoming vkd3d_shader_instruction IR to the shader model 6 pattern where only one patch constant function is emitted. This allows generation of a single patch constant function in SPIR-V.
-- v10: vkd3d-shader/normalise: Insert hull shader control point input declarations if no control point phase is defined. vkd3d-shader/spirv: Delete an unnecessary call to spirv_compiler_get_register_id().