[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]

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@@ -20,7 +20,7 @@
 
 @g = common global i32 0, align 4
 @h = common global i32 0, align 4
- at f = common global ptr null, align 4
+ at f = common global ptr zeroinitializer, align 4
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arichardson wrote:

Why do you need to update any non-AMDGPU tests here to use zeroinitializer? Is there some pass that gives different output since it does not have the datalayout available? I feel like it would be a lot easier to read this diff if the existing `null` pointers can continue to be `null`?

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