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

[Ryotaro Kasuga via llvm-commits]

https://github.com/kasuga-fj commented:

The goal of this change is to extract the common logic, right? If so, should we return the absolute value of the APInt instead?

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