vkd3d-shader: Move used_mask fixup to vsir_transform().
By turning this into a vsir_transform normalisation pass, the d3dbc parser doens't need to know the target and always retrieves the same vsir (i.e. in the same vsir dialect).
If we also want to do this normalisation for for vkd3d_shader_scan() we could put this in vsir_program_transform_early() instead.