Merge more sibling types in TypeMerging (#5452)

TypeMerging was previously able to merge identical root types and identical
siblings that were children of a distinct type, but only when the siblings had
the same top-level structure as their parent. Improve the optimization by also
merging identical children when they have a different top-level structure from
their parent.

This solution is not fully general because it does not merge shape-refining
children of separate types that would become identical siblings only after their
parent types are merged, but that full generality would require either
modifying Valmari-Lehtinen DFA minimization or performing the DFA minimization
in a loop until reaching a fixpoint.

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