combine/dce patch for PR36003, PR42575

[Eric Botcazou]

> I can tack it onto something else, but I don't see how this would reduce
> the amount of work we need to do?

The overhead of traversing the RTL or invoking DF isn't negligible so if we 
can avoid doing it one more time...  Steven's measurements showed that the RTL 
optimizers still consume 40-45% of the compilation time at -O2.

-- 
Eric Botcazou