RFR: Backport of JDK-8209686: cleanup arguments to PhaseIdealLoop() constructor

[Roman Kennke]

I'd like to backport the change for JDK-8209686, originally done in
jdk12, to jdk11u. It cleans up some code paths in C2. It makes the
backport for Shenandoah touch much less shared code.

The original patch applies cleanly on jdk11u-dev, except for some very
minor changes that I needed to make in loopnode.hpp because surrounding
comments/code did not match.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8209686
Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8209686-jdk11u/webrev.00/
Testing: tier1, tier2

Can I please get a review?

Thanks,
Roman