[llvm] [InstCombine] Add support for GEPs in `simplifyNonNullOperand` (PR #128365)

[Alexander Kornienko via llvm-commits]

alexfh wrote:

> > Google is seeing some miscompiles that track back to this PR. I'm working on a sharable test case, but just wanted to put that out there in case someone else sees something similar.
> 
> I got an automatically reduced version of the code @Sterling-Augustine is talking about here. I'm not quite sure the interesting bits weren't distorted during reduction, but maybe this will help figuring out where the problem is: https://gcc.godbolt.org/z/YYcvG69Tf

I found a UB in the original C++ code, so maybe there's no miscompilation, just wrong user code. Nevertheless, please double-check if the input I provided is handled as intended.

https://github.com/llvm/llvm-project/pull/128365