nir: add nir_lower_constant_to_temp helper

this comes up with clc.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Konstantin Seurer <konstantin.seurer@gmail.com>
Reviewed-by: Mary Guillemard <mary.guillemard@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/32382>

src/compiler/nir/nir.h

@@ -5561,6 +5561,7 @@ void nir_fixup_deref_modes(nir_shader *shader);
 void nir_fixup_deref_types(nir_shader *shader);
 
 bool nir_lower_global_vars_to_local(nir_shader *shader);
+void nir_lower_constant_to_temp(nir_shader *shader);
 
 typedef enum {
    nir_lower_direct_array_deref_of_vec_load = (1 << 0),