Use const Stmt & inside EarlyFree.cpp

Author: alexreinking

src/EarlyFree.cpp

@@ -112,7 +112,7 @@ class FindLastUse : public IRVisitor {
 Stmt inject_marker(const Stmt &stmt, const string &func, const Stmt &last_use) {
     bool injected = false;
     return mutate_with(stmt, [&](auto *self, const Block *block) -> Stmt {
-        auto do_injection = [&](Stmt s) {
+        auto do_injection = [&](const Stmt &s) {
             if (injected) {
                 return s;
             }