[8u20] RFR (S) 8043354: OptimizePtrCompare too aggressive when allocations are present

[Vladimir Kozlov]

8u20 backport request. The fix was pushed into jdk9 week ago and nightly 
testing shows no problems.

Changes from jdk9 applied to 8u without conflicts.

http://cr.openjdk.java.net/~rasbold/8043354/webrev.00/
https://bugs.openjdk.java.net/browse/JDK-8043354
http://hg.openjdk.java.net/jdk9/hs/hotspot/rev/729d5480f7db

Thanks,
Vladimir