nir: Fix copy and pasted error message in nir_validate.

These are nir_cf_nodes, not ALU instructions.
Also, use unreachable() to preempt said review feedback.

v2: Do it right (thanks Ilia).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>

src/glsl/nir/nir_validate.c

@@ -680,8 +680,7 @@ validate_cf_node(nir_cf_node *node, validate_state *state)
       break;
 
    default:
-      assert(!"Invalid ALU instruction type");
-      break;
+      unreachable("Invalid CF node type");
    }
 }