[llvm] [SandboxIR] Fix base class of FenceInst. Verify instructions when building a BB in debug mode. (PR #108078)

[via llvm-commits]

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

Let's keep the verifier ON in DEBUG builds for now to help catch such bugs.
Looks good, thanks for the patch!

https://github.com/llvm/llvm-project/pull/108078