[llvm] [SandboxIR] Add more Instruction member functions (PR #98588)

[Thorsten Schütt via llvm-commits]

tschuett wrote:

README.md would be great, but could you put a minimal SandboxIR.rst file in https://github.com/llvm/llvm-project/tree/main/llvm/docs

https://github.com/llvm/llvm-project/pull/98588