intel/compiler: Remove more users of the MRF hack
There are only three users of the "MRF hack" remaining in the FS backend:
- emit_dummy_fs(), for overriding the FS output to
/* Everyone's favorite color */
. Nobody uses this. - The repclear shader for fast clears and medium-speed clears.
- Spilling messages on Gen7-8.
This patch series eliminates the first two, giving us a bit of incremental progress toward eliminating this ugly hack. It doesn't address the third yet.