nir: Fix end of function without return warning/error.

There always is a continue block, so let us just do unreachable.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Fixes: 8cacf38f "nir: Do not use continue block after removing it."
CC: 18.1 <mesa-stable@lists.freedesktop.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107312
(cherry picked from commit e1febbef)