RFR(S) JDK-8022284: Hide internal data structure in PhaseCFG

[Roland Westrelin]

Hi Niclas,

> A review would be much appreciated.
>
> JBS: https://jbs.oracle.com/bugs/browse/JDK-8022284
> WEBREV: http://cr.openjdk.java.net/~adlertz/JDK-8022284/webrev00/

Looks good.

While you're cleaning up this code:
In block.cpp:
331   if (cfg && bh->is_Loop() && !head()->is_Root()) {

should be cfg != NULL

In lcm.cpp, !best should be best == NULL.

In output.cpp, same thing with !pinch.

Also, in vmStructs.cpp, no need for _block_arena.

Roland.