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

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

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

I would prefer not to return the absolute value, as it would violate the meaning of the function name. There is already precedent for returning an APInt from a SCEVConstant like in SCEVPatternMatch (although it would be best not to use SCEVPatternMatch in DA).

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