We can't use temporaries to guess loop dimensions, as temporaries' bounds are
calculated from loop dimensions. 

In the union:
union
{                           
  struct { ... } scalar;                         
  struct { ... } temp;
  struct gfc_ss_info info
}                           
data;

We are currently accessing data.struct.info even in the GFC_SS_TEMP case where
it is not defined. However, the aliasing and the code interact in such a way
that the temporary is never chosen to get loop bounds; so it works.

This patch prevents accessing gfc_ss::data::info in cases it has invalid
content, so that we can update gfc_ss_info without caring about aliasing
problems.
OK?