nir: add assertions that loops don't have a Continue Construct

Hoping that I didn't miss any, this *should* add assertions
to all functions and passes which explicitly handle 'nir_loop'.

Acked-by: Faith Ekstrand <faith.ekstrand@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13962>

src/compiler/nir/nir_divergence_analysis.c

@@ -1049,6 +1049,7 @@ visit_if(nir_if *if_stmt, struct divergence_state *state)
 static bool
 visit_loop(nir_loop *loop, struct divergence_state *state)
 {
+   assert(!nir_loop_has_continue_construct(loop));
    bool progress = false;
    nir_block *loop_header = nir_loop_first_block(loop);
    nir_block *loop_preheader = nir_block_cf_tree_prev(loop_header);