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/intel/compiler/brw_vec4_nir.cpp

@@ -124,6 +124,7 @@ vec4_visitor::nir_emit_if(nir_if *if_stmt)
 void
 vec4_visitor::nir_emit_loop(nir_loop *loop)
 {
+   assert(!nir_loop_has_continue_construct(loop));
    emit(BRW_OPCODE_DO);
 
    nir_emit_cf_list(&loop->body);