freedreno/ir3: add 'high' register class

freedreno/ir3: add 'high' register class

For compute shaders, we need to be able to allocate some "high"
registers (r48.x to r55.w).  (Possibly these are global to all threads
in a warp?)  Add a new register class to handle this.

Signed-off-by: Rob Clark <robdclark@gmail.com>