RFC: eliminate some dead stores

[Joe Buck]

>   > I believe this to be the #1 performance problem in GNU C++.  Any STL-like
>   > code that uses iterators with more than one data member generates tons
>   > of dead stores.  As I've said before, the Stepanov benchmark is misleading
>   > as it only tests the cases we're good at.
> 
> If you have testcases, please post them!

Boost should has lots of them, particularly in the graph library.  This part
of Boost was originally the Generic Graph Component Library, and the GGCL
developers found that KAI C++ was vastly better than C++ on it, for
this reason.  To do as well as KAI we would have to be able to allocate
structs as seperate scalars, but just being able to kill dead stores
would help a lot.