Fixed a corner-case bug in Aug 5 commit (3daecc0e).

src/base/abc/abcAig.c

@@ -904,9 +904,12 @@ void Abc_AigReplace_int( Abc_Aig_t * pMan, Abc_Obj_t * pOld, Abc_Obj_t * pNew, i
             {
                 Abc_ObjSetReverseLevel( pFanin1, Abc_ObjReverseLevel(pOld) );
                 assert( pFanin1->fMarkB == 0 );
+                if ( !Abc_ObjIsCi(pFanin1) )
+                {
                     pFanin1->fMarkB = 1;
                     Vec_VecPush( pMan->vLevelsR, Abc_ObjReverseLevel(pFanin1), pFanin1 );
                 }
+            }
             Abc_ObjPatchFanin( pFanout, pOld, pNew );            
             continue;
         }