[llvm] [InstCombine] Follow-up to "When -A + B both have nsw flag, set nsw f… (PR #72282)

[Alexander Richardson via llvm-commits]

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

Second part of commit message should probably be something like "Use `cast` instead of `dyn_cast` for cast that cannot fail.`

https://github.com/llvm/llvm-project/pull/72282