The issue was that the deps graph relation builder assumed that all
bones that had a IK constraint on them would be evaluated. However for
targetless IK bones, only the active bone would receive updates and the
others would be skipped (as those would be treated as if the IK
constraint was disabled).
I didn't see an easy way to solve this from the depsgraph side of
things.
Instead I came up with a solution that I feel is quite strait forward
and reflects what is actually supposed to happen under the hood.
Now all targetless IK constraints are treated as disabled and will not
be added to any relations in the depsgraph.
Instead, a temporary IK constraint will be created when the bone in
question is transformed. This is basically activating "Auto IK" for the
bone while transforming.
Reviewed By: Sergey, Brecht
Differential Revision: http://developer.blender.org/D7378
9a7f5f1bb4