[NFC][ADT] Fix assert message

llvm/include/llvm/ADT/Bitfields.h

 };
 
 /// `Compressor` is used to manipulate the bits of a (possibly signed) integer
 /// type so it can be packed and unpacked into a `bits` sized integer,
 /// `Compressor` is specialized on signed-ness so no runtime cost is incurred.
 /// The `pack` method also checks that the passed in `UserValue` is valid.
 template <typename T, unsigned Bits, bool = std::is_unsigned<T>::value>
 struct Compressor {
-  static_assert(std::is_unsigned<T>::value, "T is unsigned");
+  static_assert(std::is_unsigned<T>::value, "T must be unsigned");
   using BP = BitPatterns<T, Bits>;
 
   static T pack(T UserValue, T UserMaxValue) {
     assert(UserValue <= UserMaxValue && "value is too big");
     assert(UserValue <= BP::Umax && "value is too big");
     return UserValue;
   }
 
   static T unpack(T StorageValue) { return StorageValue; }
 };
 
 template <typename T, unsigned Bits> struct Compressor<T, Bits, false> {
-  static_assert(std::is_signed<T>::value, "T is signed");
+  static_assert(std::is_signed<T>::value, "T must be signed");
   using BP = BitPatterns<T, Bits>;
 
   static T pack(T UserValue, T UserMaxValue) {
     assert(UserValue <= UserMaxValue && "value is too big");
     assert(UserValue <= T(BP::Smax) && "value is too big");
     assert(UserValue >= T(BP::Smin) && "value is too small");
     if (UserValue < 0)
       UserValue &= ~BP::SignExtend;