Avoid double-lookup in QLibraryStore::findOrCreate()

The code is in a critical section, so don't waste time traversing the
QMap twice.

Now that two previous commits have re-arranged the code such that
lookup and insertion are symmetric, we can combine them into a single
lookup using operator[].

Pick-to: 6.7 6.6 6.5
Change-Id: I4a10cece65b8c35d05a9b80967bf15d2e15bd73f
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/plugin/qlibrary.cpp

@@ -414,18 +414,15 @@ inline QLibraryPrivate *QLibraryStore::findOrCreate(const QString &fileName, con
 
     QString mapName = version.isEmpty() ? fileName : fileName + u'\0' + version;
 
-    // check if this library is already loaded
-    QLibraryPrivate *lib = data->libraryMap.value(mapName);
+    QLibraryPrivate *&lib = data->libraryMap[std::move(mapName)];
     if (lib) {
+        // already loaded
         lib->libraryRefCount.ref();
         lib->mergeLoadHints(loadHints);
     } else {
         lib = lazyNewLib();
     }
 
-    // track this library
-    data->libraryMap.insert(mapName, lib);
-
     return lib;
 }