[llvm] [SLSR] Skip candidate-basis differences when `SCEVUnknown`s differ (NFC) (PR #217451)

[Justin Fargnoli via llvm-commits]

justinfargnoli wrote:

>FYI SCEV has computeConstantDifference() to determine whether there is a constant diff between values that is much faster than getMinusSCEV().

Thanks for the pointer. This looks quite interesting. I'll follow up on this shortly. 

https://github.com/llvm/llvm-project/pull/217451