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