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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> This doesn't look right. Pointers can have contain an all-zero value no matter what the sentinel value is.
> 
> I think you want to modify NullPointerIsDefined?

There are a few pieces I think need to change. I think we need a variant of isKnownNonZero that specifically checks for known-non-null pointers. Currently some of the uses of this are using as a null check.

Additionally we need to eventually do something about ConstantPointerNull, where you should be able to assume the bitvalue is what the datalayout says 

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