nir: fix sorting before assigning varying driver locations

nir: fix sorting before assigning varying driver locations

We need to make sure we also properly sort varyings sharing a single
slot otherwise we can end up assigning earlier components to the next
slot if we have already processed later components.

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/6392

Fixes: 1e93b0caa10d ("mesa/st: add support for NIR as possible driver IR")

Reviewed-by: Emma Anholt <emma@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16208>