QFlatMap: avoid post-(in|de)crement on iterators

Generic code needs to avoid post-increment and -decrement, because
they're less efficient than pre-increment and -decrement when
implemented in class types as opposed to scalars (extra copy
necessary).

Use the common pattern of implementing post- using pre-increment.

Pick-to: 6.3
Change-Id: Ida38df04c6c6f877453e5b3a1983457cc7f63f11
Reviewed-by: Jörg Bornemann <joerg.bornemann@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

src/corelib/tools/qflatmap_p.h

@@ -191,7 +191,7 @@ public:
         {
 
             iterator r = *this;
-            i++;
+            ++*this;
             return r;
         }
 
@@ -204,7 +204,7 @@ public:
         iterator operator--(int)
         {
             iterator r = *this;
-            i--;
+            --*this;
             return r;
         }
 
@@ -328,7 +328,7 @@ public:
         {
 
             const_iterator r = *this;
-            i++;
+            ++*this;
             return r;
         }
 
@@ -341,7 +341,7 @@ public:
         const_iterator operator--(int)
         {
             const_iterator r = *this;
-            i--;
+            --*this;
             return r;
         }