[PATCH] Fix optimization regression in constant folder

[Richard Kenner]

> It doesn't, though.  Having TYPE_OVERFLOW_WRAPS be true for a type
> does not cause gcc to avoid any optimizations.

Forever or just currently?  That's my concern.

I'm not trying to figure out a way to make things work now, but to establish
a framework for the future.  The semantics of sizetypes mean "don't
suppress *any* optimizations".  TYPE_OVERFLOW_WRAPS has different semantics.
Right now the implementation of those are the same.  But do we want to
guarantee that by defining things that way?