[llvm] [SCEV] Add initial matchers for SCEV expressions. (NFC) (PR #119390)

[Florian Hahn via llvm-commits]

fhahn wrote:

Rebased after https://github.com/llvm/llvm-project/pull/119389

Also added some unary matchers to make use of the flexibility of `SCEVExpr_match`

https://github.com/llvm/llvm-project/pull/119390