RFC: loopnode.cpp clean up

[Roland Westrelin]

http://cr.openjdk.java.net/~roland/shenandoah/gcinterface-loopopts/webrev.00/

This change includes some more gc interface entry point that I'd like to
propose upstream.

The one for barrier expansion from:

http://mail.openjdk.java.net/pipermail/shenandoah-dev/2018-November/008288.html

is included.

There are also a couple for loop opts so we can call our own
passes.

Also loopnode.cpp currently includes tweaks to loop strip mining
verification code. Strip mined loops are only fully built late in the
optimization process and the expectation is that we don't run loop opts
after that but with shenandoah we do. This removes most of the changes
in loopnode.cpp related to loop strip mining. I intend to propose the
only remaining change.

Comments?

Roland.