value range propagation for _bitwise_ OR

[bearophile]

Fawzi Mohamed:

> Please note that I (and probably you too) use overflow very often when  
> you subtract unsigned values.

I think it can be useful to have two flags: one for signed integer overflows only, and one for both signed&unsigned.


> D should also introduce the checked and uncheked statements to use it.

Delphi too has something similar, like {$OVERFLOWCHECKS ON} 

Bye,
bearophile