RFR: JDK-8210187: Explicit barriers for C2

[Roman Kennke]

In order to support GCs like Shenandoah, we need some explicit read- or
write-barriers in a few places. We already introduced them in the
runtime, interpreter and C1, this change intends to do the same for C2.

It follows pretty much the same design as we did in C1.

It is a bit more involved because there are *much* more intrinsics in C2
land. Other than that it's pretty straightforward.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8210187
Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8210187/webrev.00/

Can I please get a review?

Thanks,
Roman

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