[NFC][AutoUpgrade] Use ConstantPointerNull::get instead of Constant::getNullValue for known pointer types (#139984)

This is a preparation change for upcoming PRs that will update the
semantics of
`ConstantPointerNull`, making it to represent an actual `nullptr` rather
than a
zero-valued pointer.

Author: shiltian

llvm/lib/IR/AutoUpgrade.cpp

@@ -1613,7 +1613,7 @@ GlobalVariable *llvm::UpgradeGlobalVariable(GlobalVariable *GV) {
     auto Ctor = cast<Constant>(Init->getOperand(i));
     NewCtors[i] = ConstantStruct::get(EltTy, Ctor->getAggregateElement(0u),
                                       Ctor->getAggregateElement(1),
-                                      Constant::getNullValue(IRB.getPtrTy()));
+                                      ConstantPointerNull::get(IRB.getPtrTy()));
   }
   Constant *NewInit = ConstantArray::get(ArrayType::get(EltTy, N), NewCtors);
 
@@ -4721,10 +4721,10 @@ void llvm::UpgradeIntrinsicCall(CallBase *CI, Function *NewFn) {
     }
 
     // Create a new call with an added null annotation attribute argument.
-    NewCall =
-        Builder.CreateCall(NewFn, {CI->getArgOperand(0), CI->getArgOperand(1),
-                                   CI->getArgOperand(2), CI->getArgOperand(3),
-                                   Constant::getNullValue(Builder.getPtrTy())});
+    NewCall = Builder.CreateCall(
+        NewFn,
+        {CI->getArgOperand(0), CI->getArgOperand(1), CI->getArgOperand(2),
+         CI->getArgOperand(3), ConstantPointerNull::get(Builder.getPtrTy())});
     NewCall->takeName(CI);
     CI->replaceAllUsesWith(NewCall);
     CI->eraseFromParent();
@@ -4737,10 +4737,10 @@ void llvm::UpgradeIntrinsicCall(CallBase *CI, Function *NewFn) {
       return;
     }
     // Create a new call with an added null annotation attribute argument.
-    NewCall =
-        Builder.CreateCall(NewFn, {CI->getArgOperand(0), CI->getArgOperand(1),
-                                   CI->getArgOperand(2), CI->getArgOperand(3),
-                                   Constant::getNullValue(Builder.getPtrTy())});
+    NewCall = Builder.CreateCall(
+        NewFn,
+        {CI->getArgOperand(0), CI->getArgOperand(1), CI->getArgOperand(2),
+         CI->getArgOperand(3), ConstantPointerNull::get(Builder.getPtrTy())});
     NewCall->takeName(CI);
     CI->replaceAllUsesWith(NewCall);
     CI->eraseFromParent();