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

[Matt Arsenault via llvm-commits]

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@@ -306,7 +306,8 @@ struct APFloatBase {
 
 namespace detail {
 
-class IEEEFloat final : public APFloatBase {
+// Not final for testing purposes only.
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arsenm wrote:

I'd prefer to leave it as final

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