[llvm] [APFloat] Generalize bitcast to also cover x87 semantics (PR #209781)

[via llvm-commits]

janr-bay wrote:

@matthias-springer This is a follow up for https://github.com/llvm/llvm-project/pull/204200 adding the missing bitcast conversion

The tests added with that PR also cover the changed function

https://github.com/llvm/llvm-project/pull/209781