fix: optimise || expressions in template

[Rich-Harris]

small follow-up to #15056 — we can simplify the logic a tiny bit (and avoid mutating the AST node), but more to the point we can apply the same optimisation to || expressions. ultimately it would be nice to extend this to more cases (variables with a known set of possible values, ternaries, etc) but for now just picking the lowest-hanging fruit

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