[APFloat] Fix `IEEEFloat::addOrSubtractSignificand` and `IEEEFloat::normalize` (#98721) Fixes #63895 Fixes #104984 Before this PR, `addOrSubtractSignificand` presumed that the loss came from the side being subtracted, and didn't handle the case where lhs == rhs and there was loss. This can occur during FMA. This PR fixes the situation by correctly determining where the loss came from and handling it appropriately. Additionally, `normalize` failed to adjust the exponent when the significand is zero but `lost_fraction != lfExactlyZero`. This meant that the test case from #63895 was rounded incorrectly as the loss wasn't adjusted to account for the exponent being below the minimum exponent. This PR fixes this by only skipping the exponent adjustment if the significand is zero and there was no lost fraction. (Note to reviewer: I don't have commit access)
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