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

[Shilei Tian via llvm-commits]

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@@ -369,7 +378,10 @@ bool Constant::containsConstantExpression() const {
   return false;
 }
 
-/// Constructor to create a '0' constant of arbitrary type.
+/// Constructor that creates a null constant of any type. For most types, this
+/// means a constant with value '0', but for pointer types, it represents a
+/// nullptr constant. A nullptr isn't always a zero-value pointer in certain
+/// address spaces on some targets.
 Constant *Constant::getNullValue(Type *Ty) {
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shiltian wrote:

Most of the places where a `ConstantPointerNull` is constructed is via `Constant::getNull`, so if we do `Constant::getNullPtrValue`, then that will require pretty much every place to be rewritten with a `if-else` based on their type, which I tried locally to some extent a couple of months ago, and it was not fun.

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