nir: Add and use a nir_variable_list_for_mode helper

We also add a new list iterator which takes a modes bitfield and
automatically figures out which list to use.  In the future, this
iterator will work for multiple modes but today it assumes a single mode
thanks to the behavior of nir_variable_list_for_mode.  This also doesn't
work for function_temp variables.

Reviewed-by: Gert Wollny <gert.wollny@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5966>
3 files changed