[polly] [Polly] Fix assertion in addUserAssumptions for unreachable blocks (PR #227311)

[Michael Kruse via llvm-commits]

https://github.com/Meinersbur commented:

<img width="499" height="281" alt="Image" src="https://github.com/user-attachments/assets/85303c18-41bd-4c69-98cb-7c7f758814a5" />

https://github.com/llvm/llvm-project/pull/227311