broadcom/compiler: implement nir_intrinsic_load_subgroup_id correctly

broadcom/compiler: implement nir_intrinsic_load_subgroup_id correctly

For some reason, this was implemented with the bulk of the compute
shader enablement, but this intrinsic is specific to subgroups and
thus was not really used. Also, its implementation was not correct,
since it was returning the element index within the subgroup, not
the subgroup index itself, which is the index of the batch in the
dispatch.

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11620>