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

[Roland Westrelin]

> Do you know why RootNode::Ideal() isn't called during IGVN pass after 
> parsing is over?

The root node would need to be enqueued in the igvn worklist during
parsing. The code pattern when an HaltNode is added to the graph is to
use add_req() which doesn't cause the root node to be put on the igvn
worklist.

Roland.