clusterlin: make MergeSequence take SetIdx (simplification)

diff --git a/src/cluster_linearize.h b/src/cluster_linearize.h
index 58a7b8b1..47caf1f3 100644
--- a/src/cluster_linearize.h
+++ b/src/cluster_linearize.h
@@ -821,8 +821,9 @@ private:
         return child_chunk_idx;
     }
 
-    /** Make a specified active dependency inactive. */
-    void Deactivate(TxIdx parent_idx, TxIdx child_idx) noexcept
+    /** Make a specified active dependency inactive. Returns the created parent and child chunk
+     *  indexes. */
+    std::pair<SetIdx, SetIdx> Deactivate(TxIdx parent_idx, TxIdx child_idx) noexcept
     {
         // Gather and check information about the parent transactions.
         auto& parent_data = m_tx_data[parent_idx];
@@ -853,6 +854,8 @@ private:
         // Compute the new sets of reachable transactions for each new chunk.
         m_reachable[child_chunk_idx] = GetReachable(bottom_info.transactions);
         m_reachable[parent_chunk_idx] = GetReachable(top_info.transactions);
+        // Return the two new set idxs.
+        return {parent_chunk_idx, child_chunk_idx};
     }
 
     /** Activate a dependency from the bottom set to the top set. Return the index of the merged
@@ -961,11 +964,11 @@ private:
         return chunk_idx;
     }
 
-    /** Perform an upward or downward merge sequence on the specified transaction. */
+    /** Perform an upward or downward merge sequence on the specified chunk. */
     template<bool DownWard>
-    void MergeSequence(TxIdx tx_idx) noexcept
+    void MergeSequence(SetIdx chunk_idx) noexcept
     {
-        auto chunk_idx = m_tx_data[tx_idx].chunk_idx;
+        Assume(m_chunk_idxs[chunk_idx]);
         while (true) {
             auto merged_chunk_idx = MergeStep<DownWard>(chunk_idx);
             if (merged_chunk_idx == INVALID_SET_IDX) break;
@@ -981,7 +984,7 @@ private:
     {
         // Deactivate the specified dependency, splitting it into two new chunks: a top containing
         // the parent, and a bottom containing the child. The top should have a higher feerate.
-        Deactivate(parent_idx, child_idx);
+        auto [parent_chunk_idx, child_chunk_idx] = Deactivate(parent_idx, child_idx);
 
         // At this point we have exactly two chunks which may violate topology constraints (the
         // parent chunk and child chunk that were produced by deactivation). We can fix
@@ -990,9 +993,10 @@ private:
 
         // Merge the top chunk with lower-feerate chunks it depends on (which may be the bottom it
         // was just split from, or other pre-existing chunks).
-        MergeSequence<false>(parent_idx);
-        // Merge the bottom chunk with higher-feerate chunks that depend on it.
-        MergeSequence<true>(child_idx);
+        MergeSequence<false>(parent_chunk_idx);
+        // Merge the bottom chunk with higher-feerate chunks that depend on it (if it wasn't merged
+        // with the top already).
+        if (m_chunk_idxs[child_chunk_idx]) MergeSequence<true>(child_chunk_idx);
     }
 
     /** Determine the next chunk to optimize, or INVALID_SET_IDX if none. */
@@ -1242,9 +1246,7 @@ public:
         }
 
         // Otherwise, deactivate the dependency that was found.
-        Deactivate(candidate_dep.first, candidate_dep.second);
-        auto parent_chunk_idx = m_tx_data[candidate_dep.first].chunk_idx;
-        auto child_chunk_idx = m_tx_data[candidate_dep.second].chunk_idx;
+        auto [parent_chunk_idx, child_chunk_idx] = Deactivate(candidate_dep.first, candidate_dep.second);
         // Try to activate a dependency between the new bottom and the new top (opposite from the
         // dependency that was just deactivated).
         auto merged_chunk_idx = MergeChunks(child_chunk_idx, parent_chunk_idx);