PartialExecuter: Avoid copying subsets in SplitIntoSCCS

Instead of constructing a subset and then copying it into subsets,
we can construct it locally and std::move it into subsets. This is
safe because the subset has no usage outside this scope, and all
the places where we were using it now refer to the moved-subset (subsets.back())

Author: Andres Madrid Ucha

llvm/lib/CheerpWriter/PartialExecuter.cpp

@@ -1668,7 +1668,6 @@ void BasicBlockGroupNode::splitIntoSCCs(std::list<BasicBlockGroupNode>& queueToB
 		queueToBePopulated.emplace_back(data, this, blocks);
 		blockToGroupMap[bb] = &queueToBePopulated.back();
 		blockToGroupMap[start] = &queueToBePopulated.front();
-		subsetIndex[start] = 0;
 		return;
 	}
 
@@ -1685,10 +1684,10 @@ void BasicBlockGroupNode::splitIntoSCCs(std::list<BasicBlockGroupNode>& queueToB
 			subset.insert(bb);
 			subsetIndex[bb] = nextId;
 		}
-		subsets.push_back(subset);
+		subsets.push_back(std::move(subset));
 		nextId++;
-		queueToBePopulated.emplace_back(data, this, subset);
-		for (BasicBlock* bb : subset)
+		queueToBePopulated.emplace_back(data, this, subsets.back());
+		for (BasicBlock* bb : subsets.back())
 		{
 			blockToGroupMap[bb] = &queueToBePopulated.back();
 		}