RFA (S) 8175887: C1 value numbering handling of Unsafe.get*Volatile is incorrect

[Aleksey Shipilev]

Hi,

Please approve the backport of this important bugfix to 8u:

Bug:
  https://bugs.openjdk.java.net/browse/JDK-8175887

JDK 9 changeset:
  http://hg.openjdk.java.net/jdk9/hs/hotspot/rev/2ff05d967fb2

Compiler folks agree with backport, and nightlies seem clean:
  http://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/2017-March/025767.html

JDK 8u webrev:
  http://cr.openjdk.java.net/~shade/8175887/webrev.04.8u/

Product change applies cleanly, but test needs to change j.i.m.Unsafe to
s.m.Unsafe.

Testing: Linux x86_64/release hotspot/test/compiler/c1 before/after the fix.

Need a sponsor to push.

Thanks,
-Aleksey