pan/midgard: Sketch static analysis to uniform count

pan/midgard: Sketch static analysis to uniform count

This one is a little tricky, but the idea is that:

   r16-r23 are always uniforms

   r8-r15 are sometimes work, sometimes uniforms...
      ...but as work, they are always written before use
      ...and as uniforms, they are never written before use

So we use that heuristic to determine the count to feed the machine.
We'll record work register use in the next commit.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>