[llvm] [NFC] Prefer compile-time branching in function template (PR #203698)

[via llvm-commits]

https://github.com/StepfenShawn approved this pull request.

LGTM, thanks for the NFC cleanup! Using `if constexpr` here is a nice improvement to leverage compile-time branching.

https://github.com/llvm/llvm-project/pull/203698