RFR: Upstream merge to jdk-12+15

[Roman Kennke]

Merge up to jdk-12+15 brings goodies:

- 8211219: Type inconsistency in LIRGenerator::atomic_cmpxchg(..)
- 8211792: Fix misplaced BarrierSet forward declarations
- 8211279: Verify missing object equals barriers
- 8211270: GC abstraction to get real object and headers size
- 8211232: GraphKit::make_runtime_call() sometimes attaches wrong memory
state to call

Some notes:
- oopDesc::unsafe_equals() is now called oopDesc::equals_raw()
- I discovered missing direct binding from equals_raw() to actual raw
implementation. There's some boilerplate stuff that I will upstream ASAP.

Other than that: hurra! further reduced upstream diff.

Testing: tier3_gc_shenandoah

http://cr.openjdk.java.net/~rkennke/upstream-jdk12-merge-2018-10-11/outgoing.txt

Ok?

Roman