[Support] Simplify isInt and isUInt with constexpr if (NFC)

[tschuett]

llvm/include/llvm/Support/MathExtras.h

367

You can use [[maybe_unused]] instead if you like.

[craig.topper]

llvm/include/llvm/Support/MathExtras.h

367

Does it warn if you use elses? I know there was some constexpr if comments added to the no else after return guidance here https://llvm.org/docs/CodingStandards.html#don-t-use-else-after-a-return

367

Nevermind that probably won't help.

[kazu]

I'd appreciate a review. Thanks!

llvm/include/llvm/Support/MathExtras.h

 
 /// Make a 64-bit integer from a high / low pair of 32-bit integers.
 constexpr inline uint64_t Make_64(uint32_t High, uint32_t Low) {
   return ((uint64_t)High << 32) | (uint64_t)Low;
 }
 
 /// Checks if an integer fits into the given bit width.
 template <unsigned N> constexpr inline bool isInt(int64_t x) {
-  return N >= 64 || (-(INT64_C(1)<<(N-1)) <= x && x < (INT64_C(1)<<(N-1)));
-}
-// Template specializations to get better code for common cases.
-template <> constexpr inline bool isInt<8>(int64_t x) {
+  if constexpr (N == 8)
     return static_cast<int8_t>(x) == x;
-}
-template <> constexpr inline bool isInt<16>(int64_t x) {
+  if constexpr (N == 16)
     return static_cast<int16_t>(x) == x;
-}
-template <> constexpr inline bool isInt<32>(int64_t x) {
+  if constexpr (N == 32)
     return static_cast<int32_t>(x) == x;
+  if constexpr (N < 64)
+    return -(INT64_C(1) << (N - 1)) <= x && x < (INT64_C(1) << (N - 1));
+  (void)x; // MSVC v19.25 warns that x is unused.
+  return true;
 }
 
 /// Checks if a signed integer is an N bit number shifted left by S.
 template <unsigned N, unsigned S>
 constexpr inline bool isShiftedInt(int64_t x) {
   static_assert(
       N > 0, "isShiftedInt<0> doesn't make sense (refers to a 0-bit number.");
   static_assert(N + S <= 64, "isShiftedInt<N, S> with N + S > 64 is too wide.");
   return isInt<N + S>(x) && (x % (UINT64_C(1) << S) == 0);
 }
 
 /// Checks if an unsigned integer fits into the given bit width.
-///
-/// This is written as two functions rather than as simply
-///
-///   return N >= 64 || X < (UINT64_C(1) << N);
-///
-/// to keep MSVC from (incorrectly) warning on isUInt<64> that we're shifting
-/// left too many places.
-template <unsigned N>
-constexpr inline std::enable_if_t<(N < 64), bool> isUInt(uint64_t X) {
+template <unsigned N> constexpr inline bool isUInt(uint64_t x) {
   static_assert(N > 0, "isUInt<0> doesn't make sense");
-  return X < (UINT64_C(1) << (N));
-}
-template <unsigned N>
-constexpr inline std::enable_if_t<N >= 64, bool> isUInt(uint64_t) {
-  return true;
-}
-
-// Template specializations to get better code for common cases.
-template <> constexpr inline bool isUInt<8>(uint64_t x) {
+  if constexpr (N == 8)
     return static_cast<uint8_t>(x) == x;
-}
-template <> constexpr inline bool isUInt<16>(uint64_t x) {
+  if constexpr (N == 16)
     return static_cast<uint16_t>(x) == x;
-}
-template <> constexpr inline bool isUInt<32>(uint64_t x) {
+  if constexpr (N == 32)
     return static_cast<uint32_t>(x) == x;
+  if constexpr (N < 64)
+    return x < (UINT64_C(1) << (N));
+  (void)x; // MSVC v19.25 warns that x is unused.
+  return true;
 }
 
 /// Checks if a unsigned integer is an N bit number shifted left by S.
 template <unsigned N, unsigned S>
 constexpr inline bool isShiftedUInt(uint64_t x) {
   static_assert(
       N > 0, "isShiftedUInt<0> doesn't make sense (refers to a 0-bit number)");
   static_assert(N + S <= 64,