message: Remove unnecessary allocs from copy_from
diff: @@ -497,15 +497,9 @@ class EnablePolymorphicAssignment : public ConvertibleTo<ResultType> {
  public:
  using result_type = ResultType;
  - void convert_to(result_type *result) const override
  - {
  - *result = static_cast<result_type>(*self());
  - }
  + void convert_to(result_type *result) const override { *result = *self(); }
  - void move_to(result_type *result) override
  - {
  - *result = static_cast<result_type>(std::move(*self()));
  - }
  + void move_to(result_type *result) override { *result = std::move(*self()); }
  private:
  GKO_ENABLE_SELF(ConcreteType);
  