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

[Thorsten Schütt via llvm-commits]

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@@ -1796,35 +1797,55 @@ lostFraction IEEEFloat::addOrSubtractSignificand(const IEEEFloat &rhs,
   /* Subtraction is more subtle than one might naively expect.  */
   if (subtract) {
     IEEEFloat temp_rhs(rhs);
+    auto lost_fraction_is_from_rhs = false;
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tschuett wrote:

Why `auto`? `bool` has the same number of characters.

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