Remove broken assert

[r28379]

ir/opt/combo.c

@@ -307,7 +307,6 @@ static void check_opcode(const partition_t *Z)
 			first = 0;
 		} else {
 			assert((unsigned)key.code  == get_irn_opcode(irn));
-			assert(key.mode  == get_irn_mode(irn));
 			assert(key.arity == get_irn_arity(irn));
 
 			switch (get_irn_opcode(irn)) {