fix reduce computed

This fixes computed properties that depend on other computed properties.
issue: https://github.com/emberjs/ember.js/issues/4941
Maybe would fix other issues.
Or introduce more... didnt run the tests yet.

From what i saw through debuggin is that `cacheSet`
is useless. It will just be set to `undefined` when calling `propertyDidChange`

also fixes: #9313
http://emberjs.jsbin.com/hotilafamu/1/edit?html,css,js,console,output