Although it would be nice to enforce `AST_Node` cloning during transformation, that ship has sailed a long time ago.
We now get the assigned value when resolving `AST_SymbolRef` instead of `reset_opt_flags()`, which has the added advantage of improved compressor efficiency.
fixes#1787
`reduce_vars` enables substitution of variables but did not clone the value's `AST_Node`.
This confuses `collapse_vars` and result in invalid AST and subsequent crash.
fixes#1609