QMetaTypeFunctionRegistry: avoid double-lookup in insertIfNotContains()

Because there's no insertIfNotContains()-like functionality in QHash
(unlike std::unordered_map, where insert() doesn't overwrite an
existing entry), the code first called contains(k) and then insert(k,
~~~), causing two lookups in the case where the insertion actually
happens.

Fix by using the pattern QDuplicateTracker's QSet fall-back uses, too:
recording the size before and after the call to the indexing operator
and using a size increase as the criterion that an insertion should
happen. This reduces the number of lookups to one, at the cost of a
mapped_type default construction (which, given mapped_type is
std::function, should be cheap).

Change-Id: I24b31107b3e26f2eea2edce7b46f8cb5e7cb35bf
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/corelib/kernel/qmetatype.cpp

@@ -1560,9 +1560,11 @@ public:
     bool insertIfNotContains(Key k, const T &f)
     {
         const QWriteLocker locker(&lock);
-        if (map.contains(k))
+        const qsizetype oldSize = map.size();
+        auto &e = map[k];
+        if (map.size() == oldSize) // already present
             return false;
-        map.insert(k, f);
+        e = f;
         return true;
     }