Document the meaning of -1 for getCopyCost.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@63933 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Target/TargetRegisterInfo.h

@@ -202,7 +202,8 @@ public:
   unsigned getAlignment() const { return Alignment; }
 
   /// getCopyCost - Return the cost of copying a value between two registers in
-  /// this class.
+  /// this class. -1 means the register class is very expensive to copy e.g.
+  /// status flag register classes.
   int getCopyCost() const { return CopyCost; }
 };