Not assign a variable to itself 

This is to fix defect [MAGN-3917 Flashing SingleFunctionObject and failure to update custom node with run automatically] (http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-3917).

The root cause is AST nodes that generated from a partially applied function include a assignment which assigns preview variable to itself (i.e., ``v = v;``). 

At run time, this assignment will remove the dependency list of variable ``v``, therefore for this defect when inputs are updated, some garbage data is feed to the function. 

I log a defect for the clarify of [update semantics of assigning a variable to itself] (http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4036). But right now the workaround is to remove this redundant assignment from AST nodes.