message: Add GMRES_mixed to benchmark
diff: @@ -194,6 +194,24 @@ const std::map<std::string, std::function<std::unique_ptr<gko::LinOpFactory>(
  .with_preconditioner(give(precond))
  .on(exec);
  }},
  + {"gmres_mixed_double",
  + [](std::shared_ptr<const gko::Executor> exec,
  + std::shared_ptr<const gko::LinOpFactory> precond) {
  + return gko::solver::GmresMixed<double, double>::build()
  + .with_criteria(create_criterion(exec))
  + .with_krylov_dim_mixed(FLAGS_gmres_restart)
  + .with_preconditioner(give(precond))
  + .on(exec);
  + }},
  + {"gmres_mixed_float",
  + [](std::shared_ptr<const gko::Executor> exec,
  + std::shared_ptr<const gko::LinOpFactory> precond) {
  + return gko::solver::GmresMixed<double, float>::build()
  + .with_criteria(create_criterion(exec))
  + .with_krylov_dim_mixed(FLAGS_gmres_restart)
  + .with_preconditioner(give(precond))
  + .on(exec);
  + }},
  {"lower_trs",
  [](std::shared_ptr<const gko::Executor> exec,
  std::shared_ptr<const gko::LinOpFactory>) {
  