Store node reference in visitor's cache.

[vladaindjic]

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Hi, @igordejanovic ,

I've made the fix according to our today's discussion.

It may not be guaranteed that the node is always preserved during the whole visitor execution (e.g. if the node is not the AST node, but the tuple containing it). This is the rationale for keeping the reference to the node object in cache.

Best, Vladimir

[igordejanovic]

@vladaindjic Looks good. Thanks!