[llvm] [Support] Use "if constexpr" in shouldReverseIterate (PR #156812)

[Jakub Kuderski via llvm-commits]

kuhar wrote:

You can make it contexpr while still keeping the macro. I'm not saying the new implementation with `if constexpr` is worse but that I don't know if it's better.

https://github.com/llvm/llvm-project/pull/156812