RFR(XS): 8209686: cleanup arguments to PhaseIdealLoop() constructor

[Roland Westrelin]

http://cr.openjdk.java.net/~roland/8209686/webrev.00/

Instead of using 3 boolean arguments to PhaseIdealLoop(), I propose
using a single enum argument that designate what level of loop opts to
execute. This is another small change from the shenandoah repo where we
have a few extra ways of triggering loop opts.

Roland.