Add some sort of reassignment evaluation

[pylint-bot]

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

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Given the following code, trying to understand self._content += 'c' will result in an BinaryOperationError, because astroid will consider the very first value of self._content, even though it was reassigned later on. We should have some sort of evaluation that will know not to retrieve these kind of values.

#!python

  self._content = None
   ...
   ...
   self._content = "ceva"
   self._content += "c

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• Bitbucket: https://bitbucket.org/logilab/astroid/issue/141