RFR [8] [backport] 8220714: C2 Compilation failure when accessing off-heap memory using Unsafe

[Roman Kennke]

> I have not yet convinced myself the upstream fix is ideal, so let's do the next best thing: fix it
> for Shenandoah only. This backport also brings in the test that fails without the fix, and passes
> with it:
>   http://cr.openjdk.java.net/~shade/shenandoah/8u-8220714/webrev.01/
> 
> Once upstream fix arrives, we would ditch that new block.
> 
> Testing: hotspot_gc_shenandoah {fastdebug|release}; {x86_32, x86_64} builds

Upstream fix so far would be the same as this backport, except without
the guards?

I think this fix is the exact equivalent to what we've done in newer
versions. Looks good to me.

Roman