Merge bitcoin/bitcoin#25932: refactor: Simplify backtrack logic

81d4a2b refactor: Move feerate comparison invariant outside of the loop (yancy)
365aca4 refactor: Simplify feerate comparison statement (yancy)

Pull request description:

  This is a small nit, however I think it's more understandable to write:

  `utxo_pool.at(0).fee > utxo_pool.at(0).long_term_fee`

  vs

  `(utxo_pool.at(0).fee - utxo_pool.at(0).long_term_fee) > 0`

ACKs for top commit:
  Xekyo:
    ACK 81d4a2b
  achow101:
    ACK 81d4a2b
  aureleoules:
    ACK 81d4a2b

Tree-SHA512: 3e89377989c36716b53114fe40178261671dde5688075fab1c21ec173ac310f8c84ed6af90354d7c329176cb7262dfcaa7191fd19847d3b7147a9a10c3e31176

Author: achow101

src/wallet/coinselection.cpp

@@ -88,13 +88,15 @@ std::optional<SelectionResult> SelectCoinsBnB(std::vector<OutputGroup>& utxo_poo
     std::vector<size_t> best_selection;
     CAmount best_waste = MAX_MONEY;
 
+    bool is_feerate_high = utxo_pool.at(0).fee > utxo_pool.at(0).long_term_fee;
+
     // Depth First search loop for choosing the UTXOs
     for (size_t curr_try = 0, utxo_pool_index = 0; curr_try < TOTAL_TRIES; ++curr_try, ++utxo_pool_index) {
         // Conditions for starting a backtrack
         bool backtrack = false;
         if (curr_value + curr_available_value < selection_target || // Cannot possibly reach target with the amount remaining in the curr_available_value.
             curr_value > selection_target + cost_of_change || // Selected value is out of range, go back and try other branch
-            (curr_waste > best_waste && (utxo_pool.at(0).fee - utxo_pool.at(0).long_term_fee) > 0)) { // Don't select things which we know will be more wasteful if the waste is increasing
+            (curr_waste > best_waste && is_feerate_high)) { // Don't select things which we know will be more wasteful if the waste is increasing
             backtrack = true;
         } else if (curr_value >= selection_target) {       // Selected value is within range
             curr_waste += (curr_value - selection_target); // This is the excess value which is added to the waste for the below comparison