combine/dce patch for PR36003, PR42575

[Paolo Bonzini]

On 06/23/2010 12:41 PM, Eric Botcazou wrote:
> Reenabling pass_fast_rtl_byte_dce cannot be done without first evaluating the
> speed/performance trade-off, DF is known to be costly.

It's not these parts that are _extremely_ costly (it's only what is 
inherently quadratic, and cannot be helped without rewriting the 
algorithms to avoid that complexity).  But I agree that it should only 
be enabled at -O3.

Paolo