[PATCH] Fix optimization regression in constant folder

[Richard Kenner]

> The problem I see is that we mix pointers and sizetype variables

I think that's a bug and we try not to do that.  The idea is that you
compute the entire offset as a sizetype and then add it, ONCE, to a pointer.
Indeed we shouldn't be doing all sorts of calculations involving both, but
I don't think we do, at least not commonly, since most code is careful to
do it the above way.