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

[Aleksey Shipilev]

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

-- 
Thanks,
-Aleksey