[8u] RFA 8155635: C2: Mixed unsafe oop accesses break alias analysis

[david buck]

Hi!

Please approve the following backport from JDK 9 to JDK 8:

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

webrev:
http://cr.openjdk.java.net/~dbuck/jdk8155635_jdk8_ver01/

backport code review thread:
http://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/2018-October/031004.html

Cheers,
-Buck