[llvm] [InstCombine] Improve range computation in `foldICmpAddConstant` (PR #155096)

[Nikita Popov via llvm-commits]

https://github.com/nikic approved this pull request.

LGTM. The correctness of this code depends on trunc nuw on ConstantRange being exact, which ConstantRange APIs usually do not promise. It may make sense to document it as an API guarantee.

https://github.com/llvm/llvm-project/pull/155096