RFR: JDK-8206457: Code paths from oop_iterate() must use barrier-free access

[Roman Kennke]

 We have several code paths going out from oop_iterate() methods that
lead to GC barriers. This is not only inefficient but outright wrong.
oop_iterate() is normally used by GC and GC need to see the raw stuff,
not some resolved objects. In Shenandoah's full-GC it's fatal to attempt
to read objects's forwarding pointers, because it's temporarily pointing
to nowhere land.

I propose to selectively use _raw() variants of the various accessors
that are used on oop_iterate() paths. This means to introduce an
oopDesc::int_field_raw(). I also propose to change metadata_field()
accessors to always use raw access wholesale. This is only used to load
the Klass* field, which is immutable and thus doesn't require barriers.

The log_* statements in instanceRefKlass.inline.hpp surely don't need
barriers. I turned them into raw accessors as well.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8206457?filter=-1
Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8206457/webrev.00/

Test: passes hotspot-tier1 here.

Can I please get review?

Roman

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