[Support] Simplify reverseBits with constexpr if (NFC)

[kazu]

I'd appreciate a review. Thanks!

llvm/include/llvm/Support/MathExtras.h

 #define R6(n) R4(n), R4(n + 2 * 4), R4(n + 1 * 4), R4(n + 3 * 4)
   R6(0), R6(2), R6(1), R6(3)
 #undef R2
 #undef R4
 #undef R6
 };
 
 /// Reverse the bits in \p Val.
-template <typename T>
-T reverseBits(T Val) {
-  unsigned char in[sizeof(Val)];
-  unsigned char out[sizeof(Val)];
-  std::memcpy(in, &Val, sizeof(Val));
-  for (unsigned i = 0; i < sizeof(Val); ++i)
-    out[(sizeof(Val) - i) - 1] = BitReverseTable256[in[i]];
-  std::memcpy(&Val, out, sizeof(Val));
-  return Val;
-}
-
+template <typename T> T reverseBits(T Val) {
 #if __has_builtin(__builtin_bitreverse8)
-template<>
-inline uint8_t reverseBits<uint8_t>(uint8_t Val) {
+  if constexpr (std::is_same_v<T, uint8_t>)
     return __builtin_bitreverse8(Val);
-}
 #endif
-
 #if __has_builtin(__builtin_bitreverse16)
-template<>
-inline uint16_t reverseBits<uint16_t>(uint16_t Val) {
+  if constexpr (std::is_same_v<T, uint16_t>)
     return __builtin_bitreverse16(Val);
-}
 #endif
-
 #if __has_builtin(__builtin_bitreverse32)
-template<>
-inline uint32_t reverseBits<uint32_t>(uint32_t Val) {
+  if constexpr (std::is_same_v<T, uint32_t>)
     return __builtin_bitreverse32(Val);
-}
 #endif
-
 #if __has_builtin(__builtin_bitreverse64)
-template<>
-inline uint64_t reverseBits<uint64_t>(uint64_t Val) {
+  if constexpr (std::is_same_v<T, uint64_t>)
     return __builtin_bitreverse64(Val);
-}
 #endif
 
+  unsigned char in[sizeof(Val)];
+  unsigned char out[sizeof(Val)];
+  std::memcpy(in, &Val, sizeof(Val));
+  for (unsigned i = 0; i < sizeof(Val); ++i)
+    out[(sizeof(Val) - i) - 1] = BitReverseTable256[in[i]];
+  std::memcpy(&Val, out, sizeof(Val));
+  return Val;
+}
+
 // NOTE: The following support functions use the _32/_64 extensions instead of
 // type overloading so that signed and unsigned integers can be used without
 // ambiguity.
 
 /// Return the high 32 bits of a 64 bit value.
 constexpr inline uint32_t Hi_32(uint64_t Value) {
   return static_cast<uint32_t>(Value >> 32);
 }