RFR(S): 8202950: C2: assert(found_sfpt) failed: no node in loop that's not input to safepoint

[Roland Westrelin]

Hi Nils,

> Just add a comment on what idx==0 signifies before the first 
> if-statement. It's somewhat confusing that idx is used as edge index, 
> when it's mostly used as op index. But I won't require you to clean up 
> the entire file for this.

I'll do that before pushing. Thanks for the review.

Roland.