message: Try to avoid a segfault if we don't support a lowering.
  It can happen that a lowering for a certain type is not implemented yet.
  We should not segfault in such a case, but instead return a failure().
diff: @@ -131,6 +131,7 @@ class PointwiseToLinalgConverter : public OpConversionPattern<OpTy> {
  SmallVector<AffineMap, 2> indexing_maps(args.size() + (isLHLO ? 0 : 1),
  common_indexing_map);
  + bool failed = false;
  auto linalg_op = rewriter.create<linalg::GenericOp>(
  loc, op_result_types, inputs, output_buffers,
  /*initTensors=*/ValueRange{}, indexing_maps,
  @@ -141,8 +142,13 @@ class PointwiseToLinalgConverter : public OpConversionPattern<OpTy> {
  Value op_result = lmhlo::HloOpToStdScalarOp::map<OpTy>(
  op, body_result_types,
  llvm::to_vector<2>(args.take_front(inputs.size())), &rewriter);
  + if (op_result == nullptr) {
  + failed = true;
  + } else {
  nested_builder.create<linalg::YieldOp>(loc, op_result);
  + }
  });
  + if (failed) return failure();
  rewriter.replaceOp(op, linalg_op.getOperation()->getResults());
  return success();
  }
  