`simplifycfg<switch-to-lookup>` generates wrapping `add nsw`

[djkoloski]

Upstream Rust issue

This issue has been fixed by #65882. I'd like to merge that change into the 17.x release branch so I can integrate that fix into Rust. A local cherry-pick succeeded cleanly.

[nikic]

/cherry-pick fef8249

[llvmbot]

/branch llvm/llvm-project-release-prs/issue66396

[llvmbot]

/pull-request llvm/llvm-project-release-prs#700