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

[via llvm-commits]

beetrees wrote:

Either public accessors specifically for testing or a friend test class would also work. Is there any preference to which solution is best to use?

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