[llvm] [SandboxIR][NFC] Introduce templated CastInstImpl to simplify subclasses (PR #101427)

[via llvm-commits]

https://github.com/vporpo approved this pull request.

We may need a `Value::dump()` with a big switch case. I will work on that once I add the dump tests.
Anyway, feel free to drop the dumps, I think we can live without them for now.

https://github.com/llvm/llvm-project/pull/101427