Nir vectorize io (another take)

This series is based on the MR !265 (closed) that Timothy submitted (and closed). I've added support for merging variables that have more then just one component and also to check on a per shader stage basis whether two variables can be merged. For me it helps with my r600 work and since there I'm still busy getting the glsl-1.30 piglits passing I'm not worrying about barriers and TCS stuff just yet, and if it would be better to land this as a driver specific pass then I'll do that. Thanks for any comments.