[rtlopt] runtime iterations unrolling fix

[Zdenek Dvorak]

Hello,

> > Should not we increase exp by stride rather than by 1 a few lines below?
> Yes, I guess so.
> for (i=0;i<=4;i+=4)
> should get misscompiled by current code.

I just found out that the code was correct as it was :-( I should not
try to fix something that works...

I will correct my mistake together with the patch to improve
preconditioning for preincr loops.

Zdenek