QRegularExpression: make optimize() const

Although it may seem strange that such a method is const,
optimizing doesn't affect the user-visible part of the object.

Moreover, *not* having it const makes it asymmetrical with other
methods (such as match()) which are const, and under certain
conditions optimize as well.

Change-Id: I0cd8d4a6909d00629fcc65c1c3a1f011f31db782
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>

src/corelib/tools/qregularexpression.cpp

@@ -1731,7 +1731,7 @@ QRegularExpressionMatchIterator QRegularExpression::globalMatch(const QString &s
 
     \sa QRegularExpression::OptimizeOnFirstUsageOption
 */
-void QRegularExpression::optimize()
+void QRegularExpression::optimize() const
 {
     if (!isValid()) // will compile the pattern
         return;

src/corelib/tools/qregularexpression.h

@@ -125,7 +125,7 @@ public:
                                                 MatchType matchType       = NormalMatch,
                                                 MatchOptions matchOptions = NoMatchOption) const;
 
-    void optimize();
+    void optimize() const;
 
     static QString escape(const QString &str);