[RFC] Handle memset in DSE (take 2)

[Paolo Bonzini]

> I'd much rather have an rtx object that explicitly gave the
> number of repetitions, and that could itself have BLKmode.
> Thinking about it, we don't need to make the object constant
> either, despite what I said before.  How about:
> 
>     (repeat:BLK value count)
> 
> where VALUE and COUNT are rtxes.  If we want to support non-byte
> repeats, we could add a third length operand, to cope with the
> VOIDness of CONST_INT values.

Makes a lot of sense to me.

Paolo