[Coverage] Let `Decision` take account of expansions (#78969)

The current implementation (D138849) assumes `Branch`(es) would follow
after the corresponding `Decision`. It is not true if `Branch`(es) are
forwarded to expanded file ID. As a result, consecutive `Decision`(s)
would be confused with insufficient number of `Branch`(es).

`Expansion` will point `Branch`(es) in other file IDs if `Expansion` is
included in the range of `Decision`.

Fixes #77871

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Co-authored-by: Alan Phipps <a-phipps@ti.com>