[llvm] [LAA] Use computeConstantDifference() (PR #103725)

[Philip Reames via llvm-commits]

preames wrote:

> This improves LTO/ThinLTO compile-time for lencod by more than 10%

By any chance, did you try to determine if this was because we solved cases with constant differences faster, or if this was because we rejected cases without them sooner?  I'd tend to believe the later by default, but if it happened to be the former, that would imply optimization potentials inside SCEV.

https://github.com/llvm/llvm-project/pull/103725