Adjust reference/move nitpicks in use-def/reduce code

Use std::move() and avoid superfluous copies by assigning to
const-reference in a few places.

src/script_opt/Reduce.h

@@ -108,7 +108,7 @@ public:
 	// Tells the reducer to replace the given statement during the
 	// next reduction pass.
 	void AddStmtToReplace(const Stmt* s_old, StmtPtr s_new)
-		{ replaced_stmts[s_old] = s_new; }
+		{ replaced_stmts[s_old] = std::move(s_new); }
 
 	// Tells the reducer that it can reclaim the storage associated
 	// with the omitted statements.

src/script_opt/UseDefs.cc

@@ -194,7 +194,7 @@ UDs UseDefs::PropagateUDs(const Stmt* s, UDs succ_UDs,
 	case STMT_LIST:
 		{
 		auto sl = s->AsStmtList();
-		auto stmts = sl->Stmts();
+		const auto& stmts = sl->Stmts();
 
 		for ( int i = stmts.length(); --i >= 0; )
 			{
@@ -396,7 +396,7 @@ UDs UseDefs::PropagateUDs(const Stmt* s, UDs succ_UDs,
 		successor2[body.get()] = succ_stmt;
 		auto body_UDs = PropagateUDs(body.get(), succ_UDs, succ.get(), second_pass);
 
-		auto cond = w->Condition();
+		const auto& cond = w->Condition();
 		auto w_UDs = UD_Union(ExprUDs(cond.get()), body_UDs);
 		FoldInUDs(w_UDs, body_UDs);
 
@@ -404,7 +404,7 @@ UDs UseDefs::PropagateUDs(const Stmt* s, UDs succ_UDs,
 			{
 			// Create a successor for the cond_stmt
 			// that has the correct UDs associated with it.
-			auto c_as_s = w->ConditionAsStmt();
+			const auto& c_as_s = w->ConditionAsStmt();
 			auto c_as_s_UDs = std::make_shared<UseDefSet>(w_UDs);
 			CreateUDs(c_as_s.get(), c_as_s_UDs);
 

src/script_opt/UseDefs.h

@@ -95,7 +95,7 @@ private:
 	UDs PropagateUDs(const StmtPtr& s, UDs succ_UDs,
 				const StmtPtr& succ_stmt, bool second_pass)
 		{
-		return PropagateUDs(s.get(), succ_UDs, succ_stmt.get(),
+		return PropagateUDs(s.get(), std::move(succ_UDs), succ_stmt.get(),
 					second_pass);
 		}
 	UDs PropagateUDs(const Stmt* s, UDs succ_UDs,