nir: Add a pass to cut the trailing ends of vectors.

nir: Add a pass to cut the trailing ends of vectors.

Ideally we'd also handle unused middles of vectors and reswizzle ALU-only
uses of it so we could write fewer channels, but that's future work/

Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6050>