[llvm] [BPF] Implement `canLowerReturn` for big returns (PR #174478)

[via llvm-commits]

https://github.com/4ast requested changes to this pull request.

There is no such thing as 'internal functions'. Returning i128 is an ABI decision. Without kernel support this is no go.

https://github.com/llvm/llvm-project/pull/174478