nir: Add a find_variable_with_[driver_]location helper

We've hand-rolled this loop 10 places and those are just the ones I
found easily.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5966>
11 files changed