Clarify meaning of artificial

Author: sdesmalen-arm

llvm/include/llvm/MC/MCRegisterInfo.h

@@ -399,7 +399,9 @@ class MCRegisterInfo {
   /// Returns true if the given register is constant.
   bool isConstant(MCRegister RegNo) const { return get(RegNo).IsConstant; }
 
-  /// Returns true if the given register is artificial.
+  /// Returns true if the given register is artificial, which means it
+  /// represents a regunit that is not separately addressable but still needs to
+  /// be modelled, such as the top 16-bits of a 32-bit GPR.
   bool isArtificial(MCRegister RegNo) const { return get(RegNo).IsArtificial; }
 
   /// Return the number of registers this target has (useful for