Unused no-op operation nodes are not bound to a callback function, and have no outgoing relations. Incoming relations of such nodes are removed since ff60dd8b18ed00902e5bdfd36882072db7af8735. However, this was done too broadly, causing too many relations to be lost and indirectly linked objects to be unevaluated.

This commit introduces a DEPSOP_FLAG_FAKE_USER flag for operation nodes, which indicates they are not to be removed, even when they appear to be unused.