Cleanup: Don't search stack for cycle elements twice.

Cleanup: Don't search stack for cycle elements twice.

The common case is that there is no cycle.  In that case,
CheckDependencyCycle() searched the stack for a dupe from the back,
wouldn't find one, and return false.

If there was a cycle, it would then search again from the front
(probably because the push_back() that used to be here would invalidate
the ri iterator).

Since the push_back() is gone, just search from the front once. No
intended behavior change.