Loop unroll fixes

[Zoltan Hidvegi]

> > The standard defines the unsigned overflow as modulo 2^word_size
> > operation, but the result of signed overflow is undefined.
> 
> Ah, good; if the loop variable is signed, then you could apply your
> optimization.

Actually, what I am doing in not an optimization, my patch does not
make any loop that is correctly infinite in the current gcc version
finite.  This this fix could be applied, then the infinite loop bug
can be addressed in a different patch.  I would argue that infinite
loop is a different bug I that  did not address,  but  opinions can
differ on that.

Zoli