RFR: JDK-8213371: GC/C2 abstraction and cleanup to handle custom offset for GC memory accesses

[Roman Kennke]

 There are a few places in C2 where we need to deal with Shenandoah's
custom offset to load the forwarding pointer.

This patch:
- Adds hook and verification to allow GC to flatten/initialize an alias
type for (e.g.) fwd ptr accesses (btw: the 'to' field in compile.cpp
around the changed code is unused)
- in memnode.cpp removes the (unnecessary?) cast from ints to uints
- in type.cpp, make the offset check more explicit about offset > 0 or
the two predefined offsets that could be negative.

The latter two are related to the change insofar that they conflict with
Shenandoah's -8 offset, which would require extra handling without those
cleanups. Let me know if you want those changes separate.

Testing: hotspot/jtreg:tier1

Bug:
https://bugs.openjdk.java.net/browse/JDK-8213371
Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8213371/webrev.00/

Can I please get a review?

Roman

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