message: Fix segfault in TPUExtractHeadTailOutsideCompilation pass.
  getDefiningOp can return nullptr if Value is not the result of an Operation(for example an arg).
diff: @@ -331,7 +331,8 @@ void RemoveClusterAliasedOutputs(OpBuilder* builder,
  for (auto result :
  llvm::zip(cluster_terminator->getOperands(), cluster.getResults())) {
  Value cluster_terminator_operand = std::get<0>(result);
  - if (cluster.getOperation()->isProperAncestor(
  + if (cluster_terminator_operand.getDefiningOp() &&
  + cluster.getOperation()->isProperAncestor(
  cluster_terminator_operand.getDefiningOp())) {
  new_cluster_results.push_back(cluster_terminator_operand);
  new_cluster_result_types.push_back(cluster_terminator_operand.getType());
  