[llvm] [SandboxIR] Implement UnaryInstruction class (PR #101541)

[via llvm-commits]

vporpo wrote:

> I'm not sure this is useful. UnaryInstruction doesn't expose any new common methods for people to use, 

Yeah it's not super useful, but it is another way to check if an instruction has a single operand. I don't feel too strongly about keeping though. On the other hand it's not a huge burden to add it in either. There are just a handful of instructions that inherit from it.

https://github.com/llvm/llvm-project/pull/101541