Replace list-then-empty-check patterns with short-circuiting alternatives

[Copilot]

Several places computed a full filtered list only to check whether it was empty (or use just its first element), wasting work on the remaining elements.

Changes

• src/check.ml (×2) — label lookup in AddrOfLabel/Goto: replaced List.filter (function Label _ -> …) … :: _ with List.find_opt, which stops at the first Label match.

• src/frontc/cabs2cil.ml — unrollDesignatorForNestedAnonymous: replaced List.filter (fun fld -> fld.fname = designator) + fld :: _ match with List.exists, since the found value is not needed — only whether a matching field exists.

<issue_title>Audit for list short-circuiting opportunities</issue_title>

<issue_description>Similar to #221 look for other instances where a list is first computed to only compare it to the empty list after.</issue_description>

Comments on the Issue (you are @copilot in this section)

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Audit for list short-circuiting opportunities</issue_title>
<issue_description>Similar to #221 look for other instances where a list is first computed to only compare it to the empty list after.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Audit for list short-circuiting opportunities #223

---

📱 Kick off Copilot coding agent tasks wherever you are with GitHub Mobile, available on iOS and Android.

[michael-schwarz]

Should be considered in addition to #222.