RFR [10]: Allocation path should not touch GC barriers for metadata

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/10-alloc-barriers/webrev.01/

Found this during backports. The allocation path touches Shenandoah WB, which is bad enough in
itself. It fails with new asserts that verify class fields, because allocation path calls those
barriers on incomplete objects, for example *before* the klass is initialized.

sh/jdk11 onward do this correctly with raw accesses. We need to do the same in lower JDKs.

Testing: tier3_gc_shenandoah for sh/jdk10 and sh/jdk8u

Thanks,
-Aleksey