[clang] [lld] [llvm] [WIP][IR][Constants] Change the semantic of `ConstantPointerNull` to represent an actual `nullptr` instead of a zero-value pointer (PR #166667)

[Alexander Richardson via llvm-commits]

================
@@ -25493,8 +25493,8 @@ class HorizontalReduction {
                  dbgs() << "> of " << VectorizedValue << ". (HorRdx)\n");
       if (NeedShuffle)
         VectorizedValue = Builder.CreateShuffleVector(
-            VectorizedValue,
-            ConstantVector::getNullValue(VectorizedValue->getType()), Mask);
+            VectorizedValue, Constant::getNullValue(VectorizedValue->getType()),
----------------
arichardson wrote:

```suggestion
            VectorizedValue, Constant::getZeroValue(VectorizedValue->getType()),
```

https://github.com/llvm/llvm-project/pull/166667