Automerge: [ADT] Simplify rotl/rotr implementations (NFC) (#164055)

This patch simplifies rotl and rotr by ANDing the rotate amount with
N - 1.  This way, we can remove the mutual dependencies and the
forward declaration of rotr.

Author: kazutakahirata

llvm/include/llvm/ADT/bit.h

@@ -336,33 +336,28 @@ template <typename T> [[nodiscard]] T bit_ceil(T Value) {
   return T(1) << llvm::bit_width<T>(Value - 1u);
 }
 
-// Forward-declare rotr so that rotl can use it.
-template <typename T, typename = std::enable_if_t<std::is_unsigned_v<T>>>
-[[nodiscard]] constexpr T rotr(T V, int R);
-
 template <typename T, typename = std::enable_if_t<std::is_unsigned_v<T>>>
 [[nodiscard]] constexpr T rotl(T V, int R) {
-  unsigned N = std::numeric_limits<T>::digits;
+  constexpr unsigned N = std::numeric_limits<T>::digits;
 
-  R = R % N;
-  if (!R)
-    return V;
+  static_assert(has_single_bit(N), "& (N - 1) is only valid for powers of two");
+  R = R & (N - 1);
 
-  if (R < 0)
-    return llvm::rotr(V, -R);
+  if (R == 0)
+    return V;
 
   return (V << R) | (V >> (N - R));
 }
 
-template <typename T, typename> [[nodiscard]] constexpr T rotr(T V, int R) {
-  unsigned N = std::numeric_limits<T>::digits;
+template <typename T, typename = std::enable_if_t<std::is_unsigned_v<T>>>
+[[nodiscard]] constexpr T rotr(T V, int R) {
+  constexpr unsigned N = std::numeric_limits<T>::digits;
 
-  R = R % N;
-  if (!R)
-    return V;
+  static_assert(has_single_bit(N), "& (N - 1) is only valid for powers of two");
+  R = R & (N - 1);
 
-  if (R < 0)
-    return llvm::rotl(V, -R);
+  if (R == 0)
+    return V;
 
   return (V >> R) | (V << (N - R));
 }