message: [MLIR] More graceful failure in MaterializeVectors
  Even though it is unexpected except in pathological cases, a nullptr clone may
  be returned. This CL handles the nullptr return gracefuly.
diff: @@ -554,12 +554,15 @@ static bool instantiateMaterialization(Instruction *inst,
  FuncBuilder b(inst);
  auto *opInst = cast<OperationInst>(inst);
  if (auto write = opInst->dyn_cast<VectorTransferWriteOp>()) {
  - instantiate(&b, write, state->hwVectorType, state->hwVectorInstance,
  - state->substitutionsMap);
  - return false;
  + auto *clone = instantiate(&b, write, state->hwVectorType,
  + state->hwVectorInstance, state->substitutionsMap);
  + return clone == nullptr;
  } else if (auto read = opInst->dyn_cast<VectorTransferReadOp>()) {
  auto *clone = instantiate(&b, read, state->hwVectorType,
  state->hwVectorInstance, state->substitutionsMap);
  + if (!clone) {
  + return true;
  + }
  state->substitutionsMap->insert(
  std::make_pair(read->getResult(), clone->getResult(0)));
  return false;
  