nir/lower_subgroups: Properly lower masks when subgroup_size == 0

nir/lower_subgroups: Properly lower masks when subgroup_size == 0

Instead of building a constant mask (which depends on knowing the
subgroup size), we build an expression.  Because the pass uses the
nir_shader_lower_instructions helper, subgroup lowering will be run on
any newly emitted instructions as well as the previously existing
instructions.  In particular, if the subgroup size is known, the newly
emitted subgroup_size intrinsic will get turned into a constant and a
later constant folding pass will clean it up.

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