Bug 1286075: allow optimization of tasks whose dependencies have not been optimized; r=armenzg

MikeLing initially did this in bug 1287018.  The intent of this conditional was
to make optimization faster by not even checking most tasks, based on the
assumption that if the prerequisite to a task has changed (for example, a
docker image or a build), then naturally we will want to execute that task.
However, as we have developed actual optimization methods, this has proven not
to be the case: we might want to optimize a test out if its inputs have not
changed, even if a new installer has been built.  Similarly, SETA may optimize
tasks out even if their inputs have changed.

MozReview-Commit-ID: LgHET3Z84GB

taskcluster/taskgraph/optimize.py

@@ -88,9 +88,6 @@ def annotate_task_graph(target_task_graph, do_not_optimize,
         replacement_task_id = None
         if label in do_not_optimize:
             optimized = False
-        # if any dependencies can't be optimized, this task can't, either
-        elif any(not t.optimized for t in dependencies):
-            optimized = False
         # Let's check whether this task has been created before
         elif existing_tasks is not None and label in existing_tasks:
             optimized = True