aco: simplify handling of discard()
- emit nir_intrinsic_discard() as p_discard_if()
- remove block_kind_discard
- merge block_kind_uses_[demote|discard_if]
Totals from 52 (0.04% of 134913) affected shaders:
CodeSize: 16900 -> 16756 (-0.85%)
Instrs: 3093 -> 3057 (-1.16%)
Latency: 53617 -> 53588 (-0.05%); split: -0.08%, +0.02%