[hash] Enable better hash for integers

https://github.com/harfbuzz/harfbuzz/pull/4228#issuecomment-1565079537

src/hb-algs.hh

@@ -314,24 +314,17 @@ struct
   template <typename T> constexpr auto
   impl (const T& v, hb_priority<2>) const HB_RETURN (uint32_t, hb_deref (v).hash ())
 
-#if 0
-  // The following, unfortunately, while keeps the probing chains short, slows
-  // down the overall hash table performance. Not because of the extra operation
-  // itself in my opinion, but something else going on that we have not been able
-  // to track down. So for now, this is disabled. Discuss:
-  // https://github.com/harfbuzz/harfbuzz/pull/4228
-
   // Horrible: std:hash() of integers seems to be identity in gcc / clang?!
   // https://github.com/harfbuzz/harfbuzz/pull/4228
-
+  //
+  // For performance characteristics see:
+  // https://github.com/harfbuzz/harfbuzz/pull/4228#issuecomment-1565079537
   template <typename T,
 	    hb_enable_if (std::is_integral<T>::value && sizeof (T) <= sizeof (uint32_t))> constexpr auto
   impl (const T& v, hb_priority<1>) const HB_RETURN (uint32_t, v * 2654435761 /* Knuh's multiplicative hash */)
-
   template <typename T,
 	    hb_enable_if (std::is_integral<T>::value && sizeof (T) > sizeof (uint32_t))> constexpr auto
   impl (const T& v, hb_priority<1>) const HB_RETURN (uint32_t, (v ^ (v >> 32)) * 2654435761 /* Knuth's multiplicative hash */)
-#endif
 
   template <typename T> constexpr auto
   impl (const T& v, hb_priority<0>) const HB_RETURN (uint32_t, std::hash<hb_decay<decltype (hb_deref (v))>>{} (hb_deref (v)))