message: [XLA] Speed up SliceTest.
  Use parameters rather than constants, because LLVM and ptxas are slow
  with large constants.
  Use iota rather than filling with random values, because the latter is
  slow.
diff: @@ -365,15 +365,18 @@ XLA_TEST_P(SliceR2Test, DoIt) {
  const R2Spec& spec = GetParam();
  Array2D<int32> input(spec.input_dim0, spec.input_dim1);
  input.FillUnique();
  + auto literal = Literal::CreateR2FromArray2DWithLayout(
  + input, LayoutUtil::MakeLayout(spec.layout));
  XlaBuilder builder(TestName());
  - auto a = builder.ConstantR2FromArray2DWithLayout<int32>(
  - input, LayoutUtil::MakeLayout(spec.layout));
  + auto a = builder.Parameter(0, literal->shape(), "p0");
  builder.Slice(a, spec.slice_starts, spec.slice_limits, spec.slice_strides);
  + TF_ASSERT_OK_AND_ASSIGN(std::unique_ptr<GlobalData> arg,
  + client_->TransferToServer(*literal));
  std::unique_ptr<Array2D<int32>> expected = ReferenceUtil::Slice2D(
  input, spec.slice_starts, spec.slice_limits, spec.slice_strides);
  - ComputeAndCompareR2<int32>(&builder, *expected, {});
  + ComputeAndCompareR2<int32>(&builder, *expected, {arg.get()});
  }
  INSTANTIATE_TEST_CASE_P(
  @@ -453,7 +456,7 @@ class SliceR4Test : public ClientLibraryTestBase,
  void Run(const R4Spec& spec) {
  Array4D<float> values(spec.input_dims[0], spec.input_dims[1],
  spec.input_dims[2], spec.input_dims[3]);
  - values.FillRandom(3.14f);
  + values.FillIota(3.14159);
  auto expected = ReferenceUtil::Slice4D(
  values, spec.slice_starts, spec.slice_limits, spec.slice_strides);
  XlaBuilder builder(TestName());
  