[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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@@ -193,9 +193,11 @@ constexpr DataLayout::PrimitiveSpec DefaultVectorSpecs[] = {
 };
 
 // Default pointer type specifications.
-constexpr DataLayout::PointerSpec DefaultPointerSpecs[] = {
+const DataLayout::PointerSpec DefaultPointerSpecs[] = {
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shiltian wrote:

I thought about that, but we might want to allow a target to specify that the `nullptr` is an arbitrary bit pattern to be more future proof such that LLVM will not try any folding regarding those casts involving `nullptr`. I added `'c'` as the flag to specify that.

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