[llvm] [WIP] Extend data layout to add sentinel pointer value for address space. (PR #83109)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm requested changes to this pull request.

This needs additional context work to be reasonably evaluated. At a minimum, we need to refactor ValueTracking's isKnownNonZero to a separate variant that specifically understands pointer validity

https://github.com/llvm/llvm-project/pull/83109