RFR (XS): 8023472: C2 optimization breaks with G1

[Vladimir Kozlov]

http://cr.openjdk.java.net/~kvn/8023472/webrev/

The load of previous value in G1 pre-barrier code (method 
GraphKit::g1_write_barrier_pre()) does not have control. It is from the 
time when G1 code was merged into Hotspot sources. I can only speculate 
that it was done to allow commoning with a preceding normal load if it 
exists and to avoid generation of 2 loads.
This problem, I think, only affects loads from an array because a load 
from a field should be guarded by NULL check of object.

The fix is to put control on this load to always generate it after if 
(marking) check. I doubt it affect performance much. If there was load 
before the value should be in cache.

Added regression test.

Thanks,
Vladimir