pan/mdg: Assert that we don't see unknown jumps

I still don't understand why we don't see continues. But in case we do, scream
loudly so it can't be fixed.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14888>

src/panfrost/midgard/midgard_compile.c

@@ -2538,8 +2538,7 @@ emit_jump(compiler_context *ctx, nir_jump_instr *instr)
         }
 
         default:
-                DBG("Unknown jump type %d\n", instr->type);
-                break;
+                unreachable("Unhandled jump");
         }
 }