Use ValueBase instead of SILInstructions for delete notifications.

This enables e.g. notifications about the removal of SILArguments.

include/swift/SIL/Notifications.h

@@ -15,7 +15,7 @@
 
 namespace swift {
 
-class SILInstruction;
+class ValueBase;
 
 /// A protocol (or inferface) for handling instruction deletion notifications.
 ///
@@ -29,7 +29,7 @@ struct DeleteNotificationHandler {
   virtual ~DeleteNotificationHandler() {}
 
   /// Handle the invalidation message for the value \p Value.
-  virtual void handleDeleteNotification(swift::SILInstruction *Instr) { }
+  virtual void handleDeleteNotification(swift::ValueBase *Value) { }
 };
 
 } // end swift namespace