[gc-improv] Copy current trunk to branch

[Richard Kenner]

> The plan is to start using scratch allocation at the start of
> try_combine, discard everything at try_combine failure (undo_all
> call). On success promote lifetime. I haven't reached combine.c yet,
> so of course this is still not a very informed plan at this point.
> 
> I don't know what the original implementation was.

Pretty close to the above.  If I recall correctly, there was a way to
mark a position in an obstack and roll back to it.  It was marked at the
start of combine and rolled back on a failure.  If there was a success,
then the new allocations were made permanent.