Function WhichPowerOf2

Bcore/src/main/cpp/base/bit_utils.h:151–156

template<typename T>
constexpr int WhichPowerOf2(T x) {
  static_assert(std::is_integral<T>::value, "T must be integral");
  // TODO: assert unsigned. There is currently many uses with signed values.
  DCHECK((x != 0) && IsPowerOfTwo(x));
  return CTZ(x);
}

// For rounding integers.
// Note: Omit the `n` from T type deduction, deduce only from the `x` argument.