[heap] Re-enable regexp flushing

It was disabled by accident when removing code flushing. A future
experiment should check whether we actually still need it.

Bug: 
Change-Id: Iab8593d982289200775f30622f7a3ce93795d03e
Reviewed-on: https://chromium-review.googlesource.com/555430Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46322}

src/heap/mark-compact.cc

@@ -1179,7 +1179,7 @@ class MarkCompactMarkingVisitor
   // we flush the code.
   static void VisitRegExpAndFlushCode(Map* map, HeapObject* object) {
     Heap* heap = map->GetHeap();
-    if (FLAG_flush_regexp_code) {
+    if (!FLAG_flush_regexp_code) {
       JSObjectVisitor::Visit(map, object);
       return;
     }