nir: rewrite varying component packing

nir: rewrite varying component packing

There are a number of reasons for the rewrite.

1. Adding support for packing tess patch varyings in a sane way.

2. Making use of qsort allowing the code to be much easier to
   follow.

3. Fixes a bug where different interp types caused component
   packing to be skipped for all varyings in some scenarios.

4. Allows us to add a crude live range analysis for deciding
   which components should be packed together. This support can
   optionally be added in a future patch.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>