[llvm] [APFloat] Fix `IEEEFloat::addOrSubtractSignificand` and `IEEEFloat::normalize` (PR #98721)

[via llvm-commits]

beetrees wrote:

I've rebased to fix the merge conflict and added the test cases suggested in #104984. I've also changed the `AddOrSubtractSignificand` unit test to use a `friend` class as this can be done with only a single line needing to be added to APFloat.h.

https://github.com/llvm/llvm-project/pull/98721