Revert "fix opt/loop_without_phi"

This reverts commit bcaf3e7900158f4fd3c64dda9ff2d11d652ec474,
because the removed assert now holds again.

ir/opt/ldstopt.c

@@ -1847,10 +1847,8 @@ static void move_loads_out_of_loops(scc *pscc, loop_env *env)
 			}
 		}
 	}
-
-	/* Within unreachable code, there might be memory loops without Phis. */
-	if (phi_list == NULL)
-		return;
+	/* no Phis no fun */
+	assert(phi_list != NULL && "DFS found a loop without Phi");
 
 	/* for now, we cannot handle more than one input (only reducible cf) */
 	if (phi_list->next != NULL)