[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]

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@@ -72,12 +72,21 @@ bool Constant::isNegativeZeroValue() const {
 }
 
 // Return true iff this constant is positive zero (floating point), negative
-// zero (floating point), or a null value.
+// zero (floating point), zero-value pointer, or a null value.
 bool Constant::isZeroValue() const {
   // Floating point values have an explicit -0.0 value.
   if (const ConstantFP *CFP = dyn_cast<ConstantFP>(this))
     return CFP->isZero();
 
+  // Zero value pointer is a constant expression of inttoptr(0).
+  if (const auto *CE = dyn_cast<ConstantExpr>(this)) {
+    if (CE->getOpcode() == Instruction::IntToPtr) {
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arichardson wrote:

I initially thought we need to check the bitwidth matches, but since inttoptr zero-extends or truncates that doesn't matter. Maybe worth adding that as a comment?

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