QStringList::join: micro-optimization

Don't call that->size() if we have already saved its value in a local temp.

Surprisingly, this saves 56B in text size even on a -O3 GCC build, proving
that the compiler does _not_ have enough context to const-fold the repeated
size() evaluation.

Change-Id: I2c41ddc4376f4e1534fc2f0d4789e42b984d3ba6
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>

src/corelib/tools/qstringlist.cpp

@@ -443,7 +443,7 @@ QString QtPrivate::QStringList_join(const QStringList *that, const QChar *sep, i
     if (totalLength == 0)
         return res;
     res.reserve(totalLength);
-    for (int i = 0; i < that->size(); ++i) {
+    for (int i = 0; i < size; ++i) {
         if (i)
             res.append(sep, seplen);
         res += that->at(i);