Save GGC garbage produced by gimplifier

[Jan Hubicka]

> Hi,
> the memory usage of varrays increased dramatically since 3.4 accounting now
> about 20% of memory overhead.  About 60% of it (at -O0) accounts to varray
> "stack" used by gimplifier that needs not to live in ggc at all.
> 
> Bootstrapped/regtessed on i686-pc-gnu-linux.
> I don't like the need to use GENERIC_PTR for trees.  An alternative is
> to add explicit ggc_free to release the varray that looks slightly
> inferrior because of the fragmentation, but if it is preffered, I can do
> that too.

To add the timmings, Zdeneks tester reports reduction from 3m22s to
3m18s on gcc modules compilation test.  It is close to noise, but it
managed to get two runs within 1sec difference.

Honza