radeonsi: postponed KILL isn't postponed anymore, but maintains WQM

radeonsi: postponed KILL isn't postponed anymore, but maintains WQM

This restores performance for the drirc workaround, i.e.
KILL_IF does:
   visible = src0 >= 0;
   kill_flag &= visible; // accumulate kills
   amdgcn_kill(wqm_vote(visible)); // kill fully dead quads only

And all helper pixels are killed at the end of the shader:
   amdgcn_kill(kill_flag);

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>