RFR(S): 8173196: [REDO] C2 does not optimize redundant memory operations with G1

[Roland Westrelin]

http://cr.openjdk.java.net/~roland/8173196/webrev.00/

Previous attempt at this was discussed here:
http://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/2016-January/021014.html

And the follow up bugs with some comments on a possible fix:

https://bugs.openjdk.java.net/browse/JDK-8172850

The new fix is very similar to the previous one. The 2 differences are:

- aarch64 code shouldn't need any change because of 8209420 ("Track
  membars for volatile accesses so they can be properly optimized")

- The membar only affects the raw memory slice which is now properly
  handled by MachNodes thanks to 8209691 ("Allow MemBar on single memory
  slice")

Roland.