Don't mark the first/last fragment of an {ib} split as FIRST/LAST_FRAGMENT_OF_ELEMENT.
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This change allows inline margins, borders, and padding to interact correctly with {ib} splits.
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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix#14030
- [X] There are tests for these changes
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