This pattern pops up a bunch and the semantics of nir_channels() aren't very convenient much of the time. Let's add a nir_trim_vector() which matches nir_pad_vector(). Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com> Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16309>