[llvm] [WIP] Consider datalayout sentinel pointer value for isKnownNonZero check (PR #91769)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> ConstantPointerNull which needs to be updated to use this change.

I think we need to introduce a new constant type, not start reinterpreting ConstantPointerNull. There will still be use for 0 initialized pointer values, and practically speaking we'll never get any migration done if we try to change the interpretation up front 



https://github.com/llvm/llvm-project/pull/91769