Redundant logical operations left after early splitting

[Paolo Bonzini]

> Putting fwprop after combine is no problem - but is too early - none of 
> the patterns would be split at that time - preventing byte level 
> propagations.

Yeah, I meant "after split" actually.

Anyway, the problem is that if the RHS becomes a constant, fwprop does 
not propagate the LHS anymore.  I can try to add some def->use propagation.

> (PS I was impressed by fwprop code, I actually stand a chance of 
> understanding some of it)

Thanks (since I and Steven Bosscher wrote it) -- but I guess that's what 
you can expect from optimization passes that can rely on a decent 
framework (simplify-rtx.c and df-*.c).

Paolo