[llvm] [SCEV] Introduce SCEVUse, use it instead of const SCEV * (NFCI) (WIP). (PR #91961)

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

I also put together a version that stores the canonical SCEV via a pointer in each SCEV expression: https://github.com/llvm/llvm-project/commit/180f8be49b3d4db744704af11ad463a3f199d25a
 

Compile-time wise this seems to fare slightly better in most cases, with stage2-O3 being +0.17%
https://llvm-compile-time-tracker.com/compare.php?from=67c0846357bcd6faca713315380f9981a805a6e5&to=180f8be49b3d4db744704af11ad463a3f199d25a&stat=instructions:u

https://github.com/llvm/llvm-project/pull/91961