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

[Zdenek Dvorak]

Hello,

> >>I have to do something like this hack (which only works if there are
> >>other statements in the block you are moving to), methinks:
> >
> >well, almost.  Given that boundary of BIND_EXPR may be in the middle of
> >a basic block, this does not always have to be exactly what you want.
> >Not that I would be aware of an easy solution.
> 
> Richard Henderson suggested we simply change the way we process the 
> VAR_DECL's so that we walk the bind_vars before walking anything.  That 
> way, we don't have to move things around.

seems like an interesting idea; a few problems:

1) optimizations may really change a scope of variables:

{
  int k;

  {
    int i = ...

    k = i;
  }

  {
    int bla = ...;
  }

  {
    int j = k;

    ...
  }
}

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.

2) That cursed extension:

{
  int z = ...;

  {
    int array[z];
  }
}

Not really sure whether the process you describe is able to handle this.
But we probably should lower this anyway.

Zdenek