RFR (11): Streamline CAS barriers, C2 ideal graph generation and matching

[Roman Kennke]

Recently, I've streamlined the C1 and backend parts of CAS. When doing
the LRB backport to 11, I noticed that should also harmonize C2 ideal
generation and matching for CASes:

http://cr.openjdk.java.net/~rkennke/jdk11-streamline-cas-c2/webrev.00/

As far as I can tell, this makes the relevant code 100% equivalent to
what we have in jdk/jdk. Some changes are required in escape.cpp and
matcher.cpp to do this, we have the exact equivalent in
ShenandoahBarrierSetC2 matcher_* and escape_* methods in jdk/jdk. This
GC interface doesn't exist in 11.

Testing: hotspot_gc_shenandoah aarch64/x86

Can I please get a review?

Roman