[Attributor][NFC] Invalid DerefState is at fixpoint

[jdoerfert]

Summary

If the DerefBytesState (and thereby the DerefState) is invalid, we
reached a fixpoint for the whole DerefState as we will not
manifest/provide information then.

[uenoku]

LGTM.

llvm/lib/Transforms/IPO/Attributor.cpp

struct DerefState : AbstractState {
   enum {
     DEREF_NONNULL = 1 << 0,
     DEREF_GLOBAL = 1 << 1,
   };
 
   /// See AbstractState::isValidState()
   bool isValidState() const override { return DerefBytesState.isValidState(); }
 
-  // See AbstractState::isAtFixpoint()
+  /// See AbstractState::isAtFixpoint()
   bool isAtFixpoint() const override {
-    return DerefBytesState.isAtFixpoint() && NonNullGlobalState.isAtFixpoint();
+    return !isValidState() || (DerefBytesState.isAtFixpoint() &&
+                               NonNullGlobalState.isAtFixpoint());
   }
 
   /// See AbstractState::indicateOptimisticFixpoint(...)
   void indicateOptimisticFixpoint() override {
     DerefBytesState.indicateOptimisticFixpoint();
     NonNullGlobalState.indicateOptimisticFixpoint();
   }