Backward compatible fix for Coq PR#13969
This makes iris go through with the modification of Coq with Reflexive having mode ! !. Sadly, Iris uses very generic rewrite rules + typeclasses + ssr, and ssr's rewrite uses RewriteRelation to detect if it should go through setoid rewrite or not, which results in failure to recognize that these general relations are supported by setoid-rewrite if RewriteRelation has mode - !, so we switch it back to mode - - for backward compatibility. We should fix ssreflect's way to recognize relations handled by setoid-rewrite in the future, rather falling back to it if the relation is not the equality type/multi-rule, but I feel the additional instance is only a minor annoyance for now.