Fix narrowing conversion in folly::popcount

Summary: [Folly] Fix narrowing conversion in `folly::popcount`.

Reviewed By: ot, nbronson

Differential Revision: D8555467

fbshipit-source-id: fdef83fc05e3e595087e7b8a3aba98400e5a954a

folly/lang/Bits.h

@@ -123,7 +123,7 @@ inline FOLLY_INTRINSIC_CONSTEXPR unsigned int findLastSet(T const v) {
   // clang-format off
   using size = index_constant<constexpr_max(sizeof(T), sizeof(U0))>;
   return v ? 1u + ((8u * size{} - 1u) ^ static_cast<unsigned int>(
-      sizeof(T) <= sizeof(U0) ? __builtin_clz(bits_to_unsigned<U1>(v)) :
+      sizeof(T) <= sizeof(U0) ? __builtin_clz(bits_to_unsigned<U0>(v)) :
       sizeof(T) <= sizeof(U1) ? __builtin_clzl(bits_to_unsigned<U1>(v)) :
       sizeof(T) <= sizeof(U2) ? __builtin_clzll(bits_to_unsigned<U2>(v)) :
       0)) : 0u;
@@ -145,7 +145,7 @@ inline FOLLY_INTRINSIC_CONSTEXPR unsigned int popcount(T const v) {
 
   // clang-format off
   return static_cast<unsigned int>(
-      sizeof(T) <= sizeof(U0) ? __builtin_popcount(bits_to_unsigned<U1>(v)) :
+      sizeof(T) <= sizeof(U0) ? __builtin_popcount(bits_to_unsigned<U0>(v)) :
       sizeof(T) <= sizeof(U1) ? __builtin_popcountl(bits_to_unsigned<U1>(v)) :
       sizeof(T) <= sizeof(U2) ? __builtin_popcountll(bits_to_unsigned<U2>(v)) :
       0);