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?

I filed 8237086 for this.

Roland.