[llvm] [InstCombine] Relax the same-underlying-object constraint for the GEP canonicalization (PR #76583)

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

> You can use a small bit width for the alive proof, requiring noundef is unnecessary: https://alive2.llvm.org/ce/z/5EjmcW
> 
> Do you actually need the ptrtoint case? The icmp one is definitely correct, the ptrtoint one may or may not be depending on what ptrtoint semantics we decide on (e.g. it would be incorrect if it has "expose" semantics).

Done.

https://github.com/llvm/llvm-project/pull/76583