[Fortran] Improvements to dependency analysis

[Paul Thomas]

Roger,

>The following patch makes some improvements to the gfortran front-end's
>dependency analysis.  
>
If I can, I will get to this in the next 24 hours.  It is an area where 
I had been meaning to get stuck in myself.  I had made some rather 
feeble forays but your contribution looks really good.

Have you cast your eyes over the commented out bit in trans-array.c 
(gfc_conv_resolve_dependencies)?  This includes, what was previously the 
only reference to gfc_is_same_range before my patch.  A bit of loop 
shifting would go down really nicely!

Also, you would make Dominique very happy with a touch of loop reversal.....

Best regards

Paul