[CallGraphUpdater] Remove dead constants before replacing a function

Dead constants might be left when a function is replaced, we can
gracefully handle this case and avoid complexity for the users who would
see an assertion otherwise.

llvm/lib/Transforms/Utils/CallGraphUpdater.cpp

@@ -106,6 +106,7 @@ void CallGraphUpdater::removeFunction(Function &DeadFn) {
 }
 
 void CallGraphUpdater::replaceFunctionWith(Function &OldFn, Function &NewFn) {
+  OldFn.removeDeadConstantUsers();
   ReplacedFunctions.insert(&OldFn);
   if (CG) {
     // Update the call graph for the newly promoted function.

llvm/unittests/Analysis/CGSCCPassManagerTest.cpp

@@ -1608,6 +1608,10 @@ TEST_F(CGSCCPassManagerTest, TestUpdateCGAndAnalysisManagerForPasses8) {
           FnF->getEntryBlock().front().moveBefore(RI);
         ASSERT_NE(FnF, nullptr);
 
+        // Create an unsused constant that is referencing the old (=replaced)
+        // function.
+        ConstantExpr::getBitCast(FnF, Type::getInt8PtrTy(FnF->getContext()));
+
         // Use the CallGraphUpdater to update the call graph.
         CallGraphUpdater CGU;
         CGU.initialize(CG, C, AM, UR);