ir3: Use nir_split_struct_vars for temporaries

This should let us lower the RT query stack to registers instead of
scratch by getting rid of the rest of the members of the ray query
struct. This gives a 24% decrease in total time for 3DMark InVitro.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/28447>

src/freedreno/ir3/ir3_nir.c

@@ -328,6 +328,7 @@ ir3_optimize_loop(struct ir3_compiler *compiler,
       progress |= OPT(s, nir_opt_find_array_copies);
       progress |= OPT(s, nir_opt_copy_prop_vars);
       progress |= OPT(s, nir_opt_dead_write_vars);
+      progress |= OPT(s, nir_split_struct_vars, nir_var_function_temp);
 
       static int gcm = -1;
       if (gcm == -1)