This generalizes the isGEPKnownNonNull rule from ValueTracking to apply when we do not know if the base is non-null, and thus need to replace one condition with another.
This is an alternative, much more aggressive, approach to the same problem as https://reviews.llvm.org/D64533.
The core notion is that since an inbounds GEP can only form null if the base pointer is null and the offset is zero. However, if the offset is non-zero, the the "inbounds" marker makes the result poison. Thus, we're free to ignore the case where the offset is non-zero. Similarly, there's no case under which a non-null base can result in a null result without generating poison.
Reviewers - I'd appreciate careful review of the reasoning here. It's subtle, and I found several bugs in early versions of this patch. I'm not at all certain there aren't some left.