[llvm] [SandboxIR] Implement UnreachableInst (PR #101856)

[via llvm-commits]

https://github.com/vporpo commented:

Hmm does the test actually pass?

For the instruction to be created as a `sanbdoxir::UnreachableInst` we need to add the following code in `Value *Context::getOrCreateValueInternal(llvm::Value *LLVMV, llvm::User *U)`:

```
  case llvm::Instruction::Unreachable: {
    auto *LLVMUnreachable = cast<llvm::UnreachableInst>(LLVMV);
    It->second = std::unique_ptr<UnreachableInst>(new UnreachableInst(LLVMUnreachable, *this));
    return It->second.get();
  }
```

https://github.com/llvm/llvm-project/pull/101856