nir: Split nir_index_vars into two functions

We also very slightly change the semantics.  It no longer is one index
per list for global variables and is a single index over-all.

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