chore: simplify css pruning slightly

[Rich-Harris]

trying to extract small changes out into separate PRs so that the eventual PR addressing #14399 doesn't contain too many changes.

In get_possible_element_siblings, if we encounter a preceding element and adjacent_only is true, we can return immediately instead of breaking. Since there's no break, prev is always undefined after the first loop has run, meaning we always need to run the second loop.

Will instamerge to unblock the other work

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[Rich-Harris]

preview: https://svelte-dev-git-preview-svelte-14432-svelte.vercel.app/

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