Return the ConstructResult block from a list of IR blocks.

def _get_construct_result(ir_blocks):
    """Return the ConstructResult block from a list of IR blocks."""
    last_block = ir_blocks[-1]
    if not isinstance(last_block, blocks.ConstructResult):
        raise AssertionError(
            u'The last IR block {} for IR blocks {} was unexpectedly not '
            u'a ConstructResult block.'.format(last_block, ir_blocks))
    return last_block