[UPSTREAM] Use if constexpr to silence compiler warnings

This only matters downstream - in the CHERI target we specialize this
function and calling it from the specialized version triggers shift count
warnings in the else case. These warnings appear to not trigger when called
directly. Fix this by using if constexpr which is safe since the functions
are C++20 only.

Author: arichardson

libcxx/include/__bit/countr.h

@@ -39,11 +39,11 @@ _LIBCPP_NODISCARD_EXT _LIBCPP_HIDE_FROM_ABI constexpr int countr_zero(_Tp __t) n
   if (__t == 0)
     return numeric_limits<_Tp>::digits;
 
-  if (sizeof(_Tp) <= sizeof(unsigned int))
+  if constexpr (sizeof(_Tp) <= sizeof(unsigned int))
     return std::__libcpp_ctz(static_cast<unsigned int>(__t));
-  else if (sizeof(_Tp) <= sizeof(unsigned long))
+  else if constexpr (sizeof(_Tp) <= sizeof(unsigned long))
     return std::__libcpp_ctz(static_cast<unsigned long>(__t));
-  else if (sizeof(_Tp) <= sizeof(unsigned long long))
+  else if constexpr (sizeof(_Tp) <= sizeof(unsigned long long))
     return std::__libcpp_ctz(static_cast<unsigned long long>(__t));
   else {
     int __ret = 0;

libcxx/include/__bit/popcount.h

@@ -36,11 +36,11 @@ int __libcpp_popcount(unsigned long long __x) _NOEXCEPT { return __builtin_popco
 
 template <__libcpp_unsigned_integer _Tp>
 _LIBCPP_NODISCARD_EXT _LIBCPP_HIDE_FROM_ABI constexpr int popcount(_Tp __t) noexcept {
-  if (sizeof(_Tp) <= sizeof(unsigned int))
+  if constexpr (sizeof(_Tp) <= sizeof(unsigned int))
     return std::__libcpp_popcount(static_cast<unsigned int>(__t));
-  else if (sizeof(_Tp) <= sizeof(unsigned long))
+  else if constexpr (sizeof(_Tp) <= sizeof(unsigned long))
     return std::__libcpp_popcount(static_cast<unsigned long>(__t));
-  else if (sizeof(_Tp) <= sizeof(unsigned long long))
+  else if constexpr (sizeof(_Tp) <= sizeof(unsigned long long))
     return std::__libcpp_popcount(static_cast<unsigned long long>(__t));
   else {
     int __ret = 0;