Specialize qMulOverflow<2>(n) as qAddOverflow(n, n)

Addition is always more efficient than multiplication.

Pick-to: 6.4
Task-number: QTBUG-104972
Change-Id: Ib945e7df88562a2f4aea00aa319ff837191b8df2
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

src/corelib/global/qnumeric.h

@@ -317,6 +317,8 @@ template <typename T, T V2> bool qMulOverflow(T v1, std::integral_constant<T, V2
 
 template <auto V2, typename T> bool qMulOverflow(T v1, T *r)
 {
+    if constexpr (V2 == 2)
+        return qAddOverflow(v1, v1, r);
     return qMulOverflow(v1, std::integral_constant<T, V2>{}, r);
 }