[flow control] Understand deletions of variables

[pylint-bot]

Originally reported by: Claudiu Popa (BitBucket: PCManticore, GitHub: @PCManticore?)

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We should understand when a variable was deleted inside a branch and used later on. This issue requires some work in pylint as well.

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• Bitbucket: https://bitbucket.org/logilab/astroid/issue/132