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. 
Can you refactor this if-else into a HandleSpecialCaseNodes(...)