nir: Add system values from ARB_shader_ballot

We already had a channel_num system value, which I'm renaming to
subgroup_invocation to match the rest of the new system values.

Note that while ballotARB(true) will return zeros in the high 32-bits on
systems where gl_SubGroupSizeARB <= 32, the gl_SubGroup??MaskARB
variables do not consider whether channels are enabled. See issue (1) of
ARB_shader_ballot.

Reviewed-by: Connor Abbott <cwabbott0@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/compiler/nir/nir_lower_system_values.c

@@ -116,6 +116,20 @@ convert_block(nir_block *block, nir_builder *b)
                            nir_load_base_instance(b));
          break;
 
+      case SYSTEM_VALUE_SUBGROUP_EQ_MASK:
+      case SYSTEM_VALUE_SUBGROUP_GE_MASK:
+      case SYSTEM_VALUE_SUBGROUP_GT_MASK:
+      case SYSTEM_VALUE_SUBGROUP_LE_MASK:
+      case SYSTEM_VALUE_SUBGROUP_LT_MASK: {
+         nir_intrinsic_op op =
+            nir_intrinsic_from_system_value(var->data.location);
+         nir_intrinsic_instr *load = nir_intrinsic_instr_create(b->shader, op);
+         nir_ssa_dest_init(&load->instr, &load->dest, 1, 64, NULL);
+         nir_builder_instr_insert(b, &load->instr);
+         sysval = &load->dest.ssa;
+         break;
+      }
+
       default:
          break;
       }