Enable SSA at -O0

[Jan Hubicka]

Hi,
to bring little more confussion into -O1 wrt -Og discussion, I should
note that the variable tracking seems to do (relatively) resonable job
here.

I had to enforce coalescing to handle somewhat extreme testcases such
as:

t()
{
  int i=0;
  /*breakpoint here*/
  for (i=0; ....
}

where the original copy of i initialized to 0 didn't get properly
tracked and the early breakpoint got wrong value.  This is something
that I would expect to break at O0 anyway, since the initial assignment
is dead.

So in this particular case I would rather look into ways improving
register tracking rather than disabling splitting of variables.
We've heard a lot about this on GCC summit so things might improve soon
;)

Honza