Fix T96585: Intersect(Knife) tool is selecting wrong edges

Regression caused by [0] which flushed selection from vertices -> edges,
causing additional edges to be selected. Now selected is flushed based
on the mode, instead of all elements. Note that these function names
could be improved to make it clearer how these flushing functions are
different.

Also skip flushing unless selection is performed.

[0]: 55c82d8380

source/blender/editors/mesh/editmesh_intersect.c

@@ -99,8 +99,8 @@ static void edbm_intersect_select(BMEditMesh *em, struct Mesh *me, bool do_selec
           BM_edge_select_set(em->bm, e, true);
         }
       }
+      EDBM_selectmode_flush(em);
     }
-    EDBM_select_flush(em);
   }
 
   EDBM_update(me,