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

[Matt Arsenault via llvm-commits]

https://github.com/arsenm approved this pull request.

lgtm but I think there are better options than making this protected. Can the test function just be a friend, adding public accessors, or adding another base class for the tester subclass 

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