[controller] Add assertion on pipeline flush when entering debug mode

Signed-off-by: Andreas Kurth <adk@lowrisc.org>

rtl/ibex_controller.sv

@@ -997,6 +997,11 @@ module ibex_controller #(
     // If there's a pending exception req that doesn't need a PC set we must not see one
     `ASSERT(IbexNoPCSetOnSpecialReqIfNotExpected,
       exception_req_pending && !expect_exception_pc_set |-> ~pc_set_o)
+
+    // If entering or exiting debug mode, the pipeline must be flushed. This is because Ibex
+    // currently does not support some of the pipeline stages being in debug mode; either all or
+    // none of the pipeline stages must be in debug mode.
+    `ASSERT(IbexPipelineFlushOnChangingDebugMode, debug_mode_d != debug_mode_q |-> flush_id_o)
   `endif
 
   `ifdef RVFI