Tweak StringRef operators for ever-so-slightly smaller assembly

[jordan_rose]

Summary

Although StringRef::compare guarantees to return 0, 1, or -1, testing against 0 saves a single instruction in the implementation of < by replacing a cmp/sete with shr, at least when compiling with Clang. (GCC appears to figure out the idiom regardless.) It's also more obvious that the implementations are correct because the bodies match the operator names.

llvm/include/llvm/ADT/StringRef.h

#endif
 
   inline bool operator==(StringRef LHS, StringRef RHS) {
     return LHS.equals(RHS);
   }
 
   inline bool operator!=(StringRef LHS, StringRef RHS) { return !(LHS == RHS); }
 
   inline bool operator<(StringRef LHS, StringRef RHS) {
-    return LHS.compare(RHS) == -1;
+    return LHS.compare(RHS) < 0;
   }
 
   inline bool operator<=(StringRef LHS, StringRef RHS) {
-    return LHS.compare(RHS) != 1;
+    return LHS.compare(RHS) <= 0;
   }
 
   inline bool operator>(StringRef LHS, StringRef RHS) {
-    return LHS.compare(RHS) == 1;
+    return LHS.compare(RHS) > 0;
   }
 
   inline bool operator>=(StringRef LHS, StringRef RHS) {
-    return LHS.compare(RHS) != -1;
+    return LHS.compare(RHS) >= 0;
   }
 
   inline std::string &operator+=(std::string &buffer, StringRef string) {
     return buffer.append(string.data(), string.size());
   }
 
   /// @}
 
   /// Compute a hash_code for a StringRef.
   LLVM_NODISCARD
   hash_code hash_value(StringRef S);
 
 } // end namespace llvm
 
 #endif // LLVM_ADT_STRINGREF_H