nir: Add and use nir_foreach_block_unstructured helpers

These are safe to call on either structured or unstructured NIR but
don't provide the nice ordering guarantees of nir_foreach_block and
friends.  While we're here, we use them for a very small selection of
passes which are known to be safe for unstructured control-flow.  The
most important such pass is nir_dominance which is required for
structurizing.

Signed-off-by: Karol Herbst <kherbst@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/2401>