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

[Matt Arsenault via llvm-commits]

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@@ -7440,4 +7515,79 @@ TEST(APFloatTest, Float4E2M1FNToFloat) {
   EXPECT_TRUE(SmallestDenorm.isDenormal());
   EXPECT_EQ(0x0.8p0, SmallestDenorm.convertToFloat());
 }
+
+TEST(APFloatTest, AddOrSubtractSignificand) {
+  class TestIEEEFloat : detail::IEEEFloat {
+    TestIEEEFloat(bool sign, ExponentType exponent, integerPart significand)
+        : detail::IEEEFloat(1.0) {
+      // `addOrSubtractSignificand` only uses the sign, exponent and significand
+      this->sign = sign;
+      this->exponent = exponent;
+      this->significand.part = significand;
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arsenm wrote:

I don't see why this justifies adding a subclass. You can just construct the appropriate value? 

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