intel/compiler: Lower gl_SubgroupSize in postprocess_nir

intel/compiler: Lower gl_SubgroupSize in postprocess_nir

Instead of lowering the subgroup size so early, wait until we have more
information.  In particular, we're going to want different subgroup
sizes from different stages depending on the API.  We also defer
lowering of subgroup masks because the ge/gt masks require the subgroup
size to generate a subgroup mask.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>