[llvm] [SandboxIR] Implement GlobalIFunc (PR #108622)

[via llvm-commits]

vporpo wrote:

> Why does sandbox IR have to mirror something as exotic as GlobalIFunc, which should not be relevant to vectorization.

We are trying to make Sandbox IR as generic as possible, so that it can be used in other contexts if needed. GlobalIFunc and a few others are indeed quite exotic and won't probably be used. 

>  Can sandbox IR reference opaque values for things outside its scope?

It can, but we would rather not rely on it if possible. It's better to have a Sandbox IR class for something exotic, even it's missing some functionality compared to LLVM IR, rather than having an opaque object.

https://github.com/llvm/llvm-project/pull/108622