Micro-optimize QVector::count()

...by instantiating std::count() not with QVector::const_iterator, which is a
class, but with const T*, thus increasing the chance that the instantiation
can be shared with other instantiations in the executable. It might also enable
STL implementations to choose a hand-optimized version of the algorithm for C++
builtin types.

Change-Id: I93df4e58f76838d98b565f229c19e317774b7b4c
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>

src/corelib/tools/qvector.h

@@ -810,7 +810,9 @@ bool QVector<T>::contains(const T &t) const
 template <typename T>
 int QVector<T>::count(const T &t) const
 {
-    return int(std::count(cbegin(), cend(), t));
+    const T *b = d->begin();
+    const T *e = d->end();
+    return int(std::count(b, e, t));
 }
 
 template <typename T>