QRegularExpression: lock a mutex only if there's actual work to do

We can do the (atomic) test of studyData before locking the mutex
protecting the entire function body.

Change-Id: I3006e3a0028608f21668ddaebe8a799aed56362f
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qregularexpression.cpp

@@ -1137,11 +1137,11 @@ void QRegularExpressionPrivate::optimizePattern(OptimizePatternOption option)
 {
     Q_ASSERT(compiledPattern);
 
-    QMutexLocker lock(&mutex);
-
     if (studyData.load()) // already optimized
         return;
 
+    QMutexLocker lock(&mutex);
+
     if ((option == LazyOptimizeOption) && (++usedCount != qt_qregularexpression_optimize_after_use_count))
         return;