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/intel/compiler/brw_fs_nir.cpp

@@ -4103,7 +4103,7 @@ fs_visitor::nir_emit_intrinsic(const fs_builder &bld, nir_intrinsic_instr *instr
       break;
    }
 
-   case nir_intrinsic_load_channel_num: {
+   case nir_intrinsic_load_subgroup_invocation: {
       fs_reg tmp = bld.vgrf(BRW_REGISTER_TYPE_UW);
       dest = retype(dest, BRW_REGISTER_TYPE_UD);
       const fs_builder allbld8 = bld.group(8, 0).exec_all();