Defect fix: MAGN-3605 and MAGN-3646

Pre-compilation of CBN code no longer involves round-tripping from code to AST and back. Once the code is parsed into AST, it's AST all the way. Existing CBN test case regressions show no failures. A patch made to resolve a defect due to this round-tripping in AssociativeAST.cs was reverted with no side-effects. 
This isn't a great in C#, using is, immediately followed by as does a double inheritance hierachy walk.

Consider 
AssociativeNode assocNode = i as Associative node.

if (assocNode != null)
   nodes.Add(NodeUtils.Clone(assocNode));