[llvm] [DA] Let getConstantPart return optional APInt (NFC) (PR #146135)

[Ryotaro Kasuga via llvm-commits]

https://github.com/kasuga-fj approved this pull request.

I meant the motivation for this PR, which I personally think should be mentioned in the PR description.

Anyway the code itself looks good to me.

https://github.com/llvm/llvm-project/pull/146135