RFR (S) [8u] 8155635: C2: Mixed unsafe oop accesses break alias analysis

[David Buck]

Hi!

May I please get a review of this straightforward backport:

bug report:
https://bugs.openjdk.java.net/browse/JDK-8155635

JDK 9 code review thread:
http://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/2016-August/024250.html

JDK 9 changeset:
http://hg.openjdk.java.net/jdk9/jdk9/hotspot/rev/bde4bcd58309

JDK 8 webrev (for review):
http://cr.openjdk.java.net/~dbuck/jdk8155635_jdk8_ver00/

The only minor differences from the JDK 9 fix are the different assert 
message formatting [0] and the following changes to the testcase:

- use of com.oracle.java.testlibrary.Utils to get Unsafe
- addition of -XX:+UseG1GC to command line (this is default in JDK 9, so 
explicit flag was not needed)

Cheers,
-Buck

[0] https://bugs.openjdk.java.net/browse/JDK-8080775