brw: Make a helper to emit UNDEF for temporaries containing small types

Reviewed-by: Caio Oliveira <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/31833>

src/intel/compiler/brw_from_nir.cpp

@@ -2034,8 +2034,7 @@ get_nir_def(nir_to_brw_state &ntb, const nir_def &def, bool all_sources_uniform)
 
       ntb.ssa_values[def.index].is_scalar = is_scalar;
 
-      if (def.bit_size * bld.dispatch_width() < 8 * REG_SIZE)
-         bld.UNDEF(ntb.ssa_values[def.index]);
+      bld.emit_undef_for_partial_reg(ntb.ssa_values[def.index]);
 
       return ntb.ssa_values[def.index];
    } else {