QProperty: micro optimizations

- Ensure that the allocateDependencyObserver fast path is inlined.
- Use addObserver instead of observerProperty; we know that a freshly
  allocated observer does not have its prev pointer set. If prev weren't
  a private member, we could simply use Q_ASSUME(ptr->prev == nullptr),
  but making it public or befriending the class seems like a bad idea,
  as it grants too much access to the internals.

Pick-to: 6.2
Change-Id: Ia845f2807c70512563f7b9e1ecb85fe82b66208c
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/corelib/kernel/qproperty.cpp

@@ -540,7 +540,7 @@ void QPropertyBindingData::registerWithCurrentlyEvaluatingBinding_helper(Binding
 
     QPropertyObserverPointer dependencyObserver = currentState->binding->allocateDependencyObserver();
     dependencyObserver.setBindingToNotify(currentState->binding);
-    dependencyObserver.observeProperty(d);
+    d.addObserver(dependencyObserver.ptr);
 }
 
 void QPropertyBindingData::notifyObservers(QUntypedPropertyData *propertyDataPtr) const

src/corelib/kernel/qproperty_p.h

@@ -293,12 +293,17 @@ public:
             heapObservers->clear();
         dependencyObserverCount = 0;
     }
-    QPropertyObserverPointer allocateDependencyObserver()
-    {
+
+    Q_ALWAYS_INLINE QPropertyObserverPointer allocateDependencyObserver() {
         if (dependencyObserverCount < inlineDependencyObservers.size()) {
             ++dependencyObserverCount;
             return {&inlineDependencyObservers[dependencyObserverCount - 1]};
         }
+        return allocateDependencyObserver_slow();
+    }
+
+    Q_NEVER_INLINE QPropertyObserverPointer allocateDependencyObserver_slow()
+    {
         ++dependencyObserverCount;
         if (!heapObservers)
             heapObservers.reset(new std::vector<QPropertyObserver>());