[llvm] [APFloat] add power (PR #122889)

[Jakub Kuderski via llvm-commits]

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

Do you have some other usecase in mind other than https://github.com/llvm/llvm-project/pull/122450 ? If not, I don't think we should land this until we agree on that.
I'm going to add a blocking review to make sure we don't accidentally land this.

https://github.com/llvm/llvm-project/pull/122889