Extract member number *idx* from aggregate.

def extract_value(self, agg, idx, name=''):
        """
        Extract member number *idx* from aggregate.
        """
        if not isinstance(idx, (tuple, list)):
            idx = [idx]
        instr = instructions.ExtractValue(self.block, agg, idx, name=name)
        self._insert(instr)
        return instr