opt: Skip heavier UTXOs with same effective value

When two successive UTXOs differ in weight but match in effective value,
we can skip the second if the first is not selected, because all input
sets we can generate by swapping out a lighter UTXOs with a heavier UTXO
of matching effective value would be strictly worse.

Author: murchandamus

src/wallet/coinselection.cpp

@@ -479,12 +479,12 @@ util::Result<SelectionResult> CoinGrinder(std::vector<OutputGroup>& utxo_pool, c
             deselect_last();
             should_shift  = false;
 
-            // After SHIFTing to an omission branch, the `next_utxo` might have the same value and same weight as the
-            // UTXO we just omitted (i.e. it is a "clone"). If so, selecting `next_utxo` would produce an equivalent
-            // selection as one we previously evaluated. In that case, increment `next_utxo` until we find a UTXO with a
-            // differing amount or weight.
-            while (utxo_pool[next_utxo - 1].GetSelectionAmount() == utxo_pool[next_utxo].GetSelectionAmount()
-                    && utxo_pool[next_utxo - 1].m_weight == utxo_pool[next_utxo].m_weight) {
+            // After SHIFTing to an omission branch, the `next_utxo` might have the same effective value as the UTXO we
+            // just omitted. Since lower weight is our tiebreaker on UTXOs with equal effective value for sorting, if it
+            // ties on the effective value, it _must_ have the same weight (i.e. be a "clone" of the prior UTXO) or a
+            // higher weight. If so, selecting `next_utxo` would produce an equivalent or worse selection as one we
+            // previously evaluated. In that case, increment `next_utxo` until we find a UTXO with a differing amount.
+            while (utxo_pool[next_utxo - 1].GetSelectionAmount() == utxo_pool[next_utxo].GetSelectionAmount()) {
                 if (next_utxo >= utxo_pool.size() - 1) {
                     // Reached end of UTXO pool skipping clones: SHIFT instead
                     should_shift = true;