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

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

I think I'd rather just leave ConstantPointerNull as meaning the all-zero bit pattern.  Some places actually do care that it's all-zero, and there's not much benefit to changing it.  But better to discuss that on an RFC, probably.

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