rusticl: Initialize NIR option structs with default values

Eases NIR maintenance since adding new members no longer requires touching rusticl.

The one const `nir_remove_dead_variables_options` has been duplicated as two local variables because
`default()` cannot be called from const contexts, yet.

Reviewed-by: Karol Herbst <kherbst@redhat.com>
Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/35497>
1 file changed