[RFA] A small fix for IRA

[Vladimir Makarov]

Jie Zhang wrote:
> When working on a GCC bug, I found this. We should use `-' instead of 
> `+', otherwise "use + 1" would fall out of the array boundary. Is it OK?
>
>
Yes, that is definitely a typo.  So your patch is ok.  Thanks for 
finding this.

P.S.  By the way, the comparison is not necessary at all because use < 
n_operands is always true and if constraints[use][0]=='%' than use + 1 < 
n_operands.  But it is good to have it for readability.