Function RoundDown

Bcore/src/main/cpp/base/bit_utils.h:164–167

template<typename T>
constexpr T RoundDown(T x, typename Identity<T>::type n) {
  DCHECK(IsPowerOfTwo(n));
  return (x & -n);
}

template<typename T>
constexpr T RoundUp(T x, typename std::remove_reference<T>::type n) WARN_UNUSED;