[turbofan] Bring back early return in JSInliningHeuristic.

This redcues the noise created by --trace-turbo-inlining when there
actually are no candidates being processed.

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/1410723004

Cr-Commit-Position: refs/heads/master@{#31885}

src/compiler/js-inlining-heuristic.cc

@@ -89,6 +89,7 @@ Reduction JSInliningHeuristic::Reduce(Node* node) {
 
 
 void JSInliningHeuristic::Finalize() {
+  if (candidates_.empty()) return;  // Nothing to do without candidates.
   if (FLAG_trace_turbo_inlining) PrintCandidates();
 
   while (!candidates_.empty()) {