[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]

================

----------------
arichardson wrote:

```suggestion
if (V->isNullValue() && !DestTy->isX86_AMXTy())
return Constant::getNullValue(DestTy);
```
Addrspacecast was already handled above. Not sure what is special about isX86_AMXTy, but I guess we need it.

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