Revisiting the TYPE_IS_SIZETYPE issue

[Paolo Bonzini]

> So my idea (I have this for some time now, but there's always lack
> of time to implement it ...) is to make the existing tree codes
> 'may overflow' operations and introduce new tree code variants
> with the 'does not overflow' semantics.  This way we conservatively
> preserve 'may overflow' in legacy code.

That seems extremely ugly, especially to make fold-const.c aware of 
that.  It would be better to make the "overflow definition" a flag for a 
single PLUS_EXPR node (or PLUS rtl).

Paolo