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

[Nikita Popov via llvm-commits]

nikic wrote:

Mirroring unnecessary things into SandboxIR increases the burden on maintaining IR, because any change to IR has to be synced to SandboxIR. In that respect, it would be preferable if SandboxIR only mirrored the parts that are necessary to the prototype of the sandbox vectorizer. It looks like it currently tries to mirror *all* APIs of *all* types in the Value hierarchy.

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