Eliminate dead code.

def visit_If(self, node):
        """Eliminate dead code."""
        # do not optimize ifs that have a block inside so that it doesn't
        # break super().
        if node.find(nodes.Block) is not None:
            return self.generic_visit(node)
        try:
            val = self.visit(node.test).as_const()
        except nodes.Impossible:
            return self.generic_visit(node)
        if val:
            body = node.body
        else:
            body = node.else_
        result = []
        for node in body:
            result.extend(self.visit_list(node))
        return result