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

Blunder -- two threads may step into this method together,
both see not studied, and both study (with one leaking its study data).

This reverts commit 5fbd787cf9a72621d66604a4898f06ea4365226e.

Change-Id: Ia746925abcad1e43adf4f6f1d495b018de022b07
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>

src/corelib/tools/qregularexpression.cpp

@@ -1149,11 +1149,11 @@ 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;