RFR(S): 8231291: C2: loop opts before EA should maximally unroll loops

[Roland Westrelin]

> I understand that the change you have is enough to avoid the problematic 
> case in GraphKit::must_be_not_null(). What I'd like to get rid of is the 
> need for a correctness check to ensure that HaltNode can't go away. It's 
> error-prone and can eventually get out of sync.

Fair enough. Then why not always enqueue the root node for igvn at the
end of parsing?

> Also, I'd like the bug to be handled separately (from 8231291). It'll 
> simplify possible backporting: as I see from the history, the 
> problematic code (JDK-8176506) was introduced in 10 and it may show up 
> in 11u eventually.

You're right. That's cleaner. I'll send new webrevs.

Roland.