nir,intel: lower if (cond) demote() to new intrinsic demote_if(cond)

This will effectively enable the optimization in anv.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/compiler/nir/nir_gather_info.c

@@ -199,6 +199,7 @@ gather_intrinsic_info(nir_intrinsic_instr *instr, nir_shader *shader,
 {
    switch (instr->intrinsic) {
    case nir_intrinsic_demote:
+   case nir_intrinsic_demote_if:
    case nir_intrinsic_discard:
    case nir_intrinsic_discard_if:
       assert(shader->info.stage == MESA_SHADER_FRAGMENT);