[llvm] [APFloat]extract `fltSemantics::isRepresentableBy` to header (PR #122636)

[Jakub Kuderski via llvm-commits]

kuhar wrote:

@matthias-springer could you confirm if this is compatible with https://discourse.llvm.org/t/rethink-on-approach-to-low-precision-fp-types/82361 . IIUC, it should be.

https://github.com/llvm/llvm-project/pull/122636