Enable SSA at -O0

[Paolo Bonzini]

>>> ... I agree with the AdaCore guys' opinion was that in 3.x -O1 was -Og in
>>> practice, and it was then broken in 4.x. 
>> Me too.  I think that -O1 should be those optimizations that can be done
>> without undue effects on either the debug experience or on compile time.
> 
> Ok.  We should then drop some obviously debug-breaking stuff from -O1,
> like SRA.

Maybe we can throttle it down?  SRA on compiler-generated temporaries 
would be sometimes enough for C++.

Paolo