[tree-ssa] Fix variables getting out of their scope

[Zdenek Dvorak]

Hello,

> >copy propagation may replace set to j with i, thus moving it out of its
>  >scope.  Given that we may (??? not really sure whether we do it) assign
>  >the same stack slot to i, j and bla, this would cause a misscompilation.
> Everything which goes into a stack slot (as opposed to be assigned a 
> pseudo register) should be marked as addressable -- which should in turn
> prevent the object from being used for copy propagation.  ie, it should
> be a non-issue.

probably in this particular case; but it would be anoying to have to
care about that at every single optimization.

Zdenek