message: [TF:XLA] Simplify ResourceApplyAdadelta
  Use higher level functions like Sqrt and Rsqrt rather than Pow with different exponents.
diff: @@ -856,15 +856,12 @@ class ResourceApplyAdadelta : public XlaOpKernel {
  xla::XlaOp grad = ctx->Input(6);
  xla::XlaBuilder* b = ctx->builder();
  - xla::XlaOp neg_half = XlaHelpers::FloatLiteral(b, dtype_, -0.5);
  - xla::XlaOp half = XlaHelpers::FloatLiteral(b, dtype_, 0.5);
  xla::XlaOp one = XlaHelpers::FloatLiteral(b, dtype_, 1.0);
  - xla::XlaOp two = XlaHelpers::FloatLiteral(b, dtype_, 2.0);
  - accum = rho * accum + (one - rho) * xla::Pow(grad, two);
  - xla::XlaOp update = xla::Pow(accum_update + epsilon, half) *
  - xla::Pow(accum + epsilon, neg_half) * grad;
  - accum_update = rho * accum_update + (one - rho) * xla::Pow(update, two);
  + accum = rho * accum + (one - rho) * xla::Square(grad);
  + xla::XlaOp update =
  + xla::Sqrt(accum_update + epsilon) * xla::Rsqrt(accum + epsilon) * grad;
  + accum_update = rho * accum_update + (one - rho) * xla::Square(update);
  var = var - update * lr;
  OP_REQUIRES_OK(ctx, ctx->AssignVariable(0, dtype_, var));
  OP_REQUIRES_OK(ctx, ctx->AssignVariable(1, dtype_, accum));
  