[Compiler] Fast-path CompilerDispatcher::IsEnqueued when jobs_ is empty.

BUG=v8:5203

Change-Id: Iee83ac42fc17bb455ea2104540d2d3bc60260a42
Reviewed-on: https://chromium-review.googlesource.com/441705Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43194}

src/compiler-dispatcher/compiler-dispatcher.cc

@@ -356,6 +356,7 @@ bool CompilerDispatcher::EnqueueAndStep(
 bool CompilerDispatcher::IsEnabled() const { return FLAG_compiler_dispatcher; }
 
 bool CompilerDispatcher::IsEnqueued(Handle<SharedFunctionInfo> function) const {
+  if (jobs_.empty()) return false;
   return GetJobFor(function) != jobs_.end();
 }