nir: Add system values from ARB_shader_ballot

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>