RFR (11): [backport] 8221766: Load-reference barriers for Shenandoah

[Roman Kennke]

This is the backport of load-reference-barriers to sh/jdk11. It goes on
top of the recently proposed (and not-yet-pushed) CAS streamlining.

It is a tremendous harmonization in the C1 and C2 barriers sets: I
basically copied over the relevant files from jdk/jdk at the point of
the Load-ref-barriers-change, and retrofitted it to match jdk11.

I have *not* yet backported the LRB-related-bugfixes, yet. Let's start
review on this here, while I'll pile on the followup-bugfixes in the
meantime.

I also went over the sh/jdk11 vs. upstream jdk11u diff and reverted any
change that's not needed anymore (resolve() and equals() stuff for the
most part). It is *much* nicer now. :-) Notably, it makes
oopDesc::unsafe_equals() disappear, and we should do the equivalent in
jdk/jdk to match it.

http://cr.openjdk.java.net/~rkennke/backport-jdk11-JDK-8221766/webrev.00/

Testing: hotspot_gc_shenandoah x86/aarch64

What do you think?

Roman