[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 cfe-commits]

================
@@ -1543,6 +1558,13 @@ Constant *llvm::ConstantFoldCastOperand(unsigned Opcode, 
Constant *C,
     }
     break;
   case Instruction::IntToPtr:
+    // We can fold it to a null pointer if the input is the nullptr value.
+    if (std::optional<APInt> NullPtrValue = DL.getNullPtrValue(
+            DestTy->getScalarType()->getPointerAddressSpace())) {
+      if ((NullPtrValue->isZero() && C->isZeroValue()) ||
+          (NullPtrValue->isAllOnes() && C->isAllOnesValue()))
----------------
arichardson wrote:

I think we have to check the width of the input value for all ones since 
inttoptr zero-extends, so a short value would not end up as canonical nullptr. 

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