This patch fixes the update of the DAGNode UnscheduledSucc counter when
a use edge is modified. This is the result of a setOperand() or a RAUW
(and friends) operation.
Before this patch we would not check if the User (i.e., the consumer of
the use-def edge) is scheduled and we would update the definition's
UnscheduledSucc counter, resulting in counting errors.
For example, consider the following IR:
```
%A = ...
%B = ...
%U = %A ; scheduled
```
Note that %U's DAGNode is marked as "scheduled" while %A and %B are not.
If we change %U's operand from %A to %B then we should not attempt to
update %A's or %B's UnscheduledSuccs because %U is scheduled so it
should not get counted as an "unscheduled" successor.
720615f495