SSA_NAMEs not always released

[Daniel Berlin]

> If we take this after DCE, we still refer to a statement which no longer
> exists which we don't collect in the GC.
As i said when i pointed this out to you, we should probably skip
walking common.chain on SSA_NAME.
But that's non-trivial hard because it's a reused field :(

> 
> Could someone look into this and see what they can do?  

You should probably ask Diego or Andrew directly whether they'd like us
to do this in bsi_remove (which requires adding an argument) and
remove_phi_node(ditto) so that a lot less has to worry about releasing
defs.

> This will 
> improve
> memory usage in GCC and also there will be more SSA_NAME reuse which is
> good.

> 
> Thanks,
> Andrew Pinsk
> 
> 
> 
>