nir: add support for user defined select control

This will allow us to make use of the selection control support in
spirv and the GL support provided by EXT_control_flow_attributes.

Note this only supports if-statements as we dont support switches
in NIR.

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=108841

src/compiler/nir/nir_clone.c

@@ -537,6 +537,7 @@ static nir_if *
 clone_if(clone_state *state, struct exec_list *cf_list, const nir_if *i)
 {
    nir_if *ni = nir_if_create(state->ns);
+   ni->control = i->control;
 
    __clone_src(state, ni, &ni->condition, &i->condition);