RFR(M): 8244504: C2: refactor counted loop code in preparation for long counted loop

[Roland Westrelin]

Here is an updated webrev:

http://cr.openjdk.java.net/~roland/8244504/webrev.02/

I moved the min/max methods to MaxNode and filed
https://bugs.openjdk.java.net/browse/JDK-8245787

I noticed the previous webrev incorrectly included some of the changes
from 8223051 (support loops with long (64b) trip counts) and removed
them. I intend this webrev to only be refactoring in preparation for the
long loops support.

Anyone else for a review?

Roland.