[SCCP] Avoid modifying AdditionalUsers while iterating over it

When run under valgrind, or with a malloc that poisons freed memory,
this can lead to segfaults or other problems.

To avoid modifying the AdditionalUsers DenseMap while still iterating,
save the instructions to be notified in a separate SmallPtrSet, and use
this to later call OperandChangedState on each instruction.

Fixes PR49582.

Reviewed By: fhahn

Differential Revision: https://reviews.llvm.org/D98602

(cherry picked from commit 6abb92f2)

llvm/lib/Transforms/Scalar/SCCP.cpp

@@ -542,9 +542,14 @@ private:
 
     auto Iter = AdditionalUsers.find(I);
     if (Iter != AdditionalUsers.end()) {
+      // Copy additional users before notifying them of changes, because new
+      // users may be added, potentially invalidating the iterator.
+      SmallVector<Instruction *, 2> ToNotify;
       for (User *U : Iter->second)
         if (auto *UI = dyn_cast<Instruction>(U))
-          OperandChangedState(UI);
+          ToNotify.push_back(UI);
+      for (Instruction *UI : ToNotify)
+        OperandChangedState(UI);
     }
   }
   void handleCallOverdefined(CallBase &CB);