RFR(L) 8034198: Cleanup and re-factorize PhaseChaitin::build_ifg_physical()

[Niclas Adlertz]

Hi all,

This is a first step to clean up the register allocator in C2. In this change we have divided the very long method build_ifg_physical() into many smaller ones, making it easier to see the steps when building the IFG (and computing the block pressure).
We have also cleaned up old comments and improved old code, both by better naming and by simplifying expressions.
I personally think that the easiest way to review this change is to have the old and new ifg.cpp side by side, and start from build_ifg_physical().
The next step is to create a new class for these methods, isolating them and removing them from PhaseChaitin.
  
WEBREV: http://cr.openjdk.java.net/~adlertz/JDK-8031498/webrev00/
BUG: https://bugs.openjdk.java.net/browse/JDK-8031498

Kind Regards,
Niclas Adlertz