fold_convert question

[Richard Kenner]

    Solving the tree combination problem in a general way that allows
    us to remove the useless type conversion, forward propagation,
    expression reassociation and related problems should be high on
    our todo list once we change focus from 4.0 to 4.1.

I agree. But the tree combiner should also be able to compute the sorts of
data that the RTL combiner produces (which bits are known zero and how
many copies of the sign bit we know we have).  How to do this was discussed
at the last summit.