message: Add missing elementwise monotonic functions
  This PR adds `Acos`, `Acosh`, `Asin`, `Atan`, `Softsign` and `Softplus` to the collection of elementwise monotonic operations. This allows the arithmetic optimizer to optimize more cases.
  Continuation of
diff: @@ -164,17 +164,39 @@ bool IsDiv(const NodeDef& node) { return node.op() == "Div"; }
  bool IsElementWiseMonotonic(const NodeDef& node, bool* is_non_decreasing) {
  static const gtl::FlatSet<string>* const kMonotonicNonDecreasingOps =
  CHECK_NOTNULL((new gtl::FlatSet<string>{
  - "Asinh", "Atanh", "Ceil", "Elu", "Erf", "Exp", "Expm1",
  - "Floor", "Log", "Log1p", "Relu", "Relu", "Relu6", "Rint",
  - "Selu", "Sigmoid", "Sign", "Sinh", "Sqrt", "Tanh",
  + "Acosh",
  + "Asin",
  + "Asinh",
  + "Atan",
  + "Atanh",
  + "Ceil",
  + "Elu",
  + "Erf",
  + "Exp",
  + "Expm1",
  + "Floor",
  + "Log",
  + "Log1p",
  + "Relu",
  + "Relu6",
  + "Rint",
  + "Selu",
  + "Sigmoid",
  + "Sign",
  + "Sinh",
  + "Softsign",
  + "Softplus",
  + "Sqrt",
  + "Tanh",
  }));
  static const gtl::FlatSet<string>* const kMonotonicNonIncreasingOps =
  CHECK_NOTNULL((new gtl::FlatSet<string>{
  - "Inv",
  - "Reciprocal",
  + "Acos",
  "Erfc",
  - "Rsqrt",
  + "Inv",
  "Neg",
  + "Reciprocal",
  + "Rsqrt"
  }));
  if (kMonotonicNonDecreasingOps->count(node.op()) > 0) {
  if (is_non_decreasing) {
  