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

[Roland Westrelin]

https://bugs.openjdk.java.net/browse/JDK-8244504
http://cr.openjdk.java.net/~roland/8244504/webrev.00/

This is some refactoring in the counted loop code to prepare for 8223051
(support loops with long (64b) trip counts). Some of the changes came up
in the review of 8223051 (that patch used to be part of 8223051).

Roland.