[8u40] Backport 8030976: Untaken paths should be more vigorously pruned at highest optimization level

[Rickard Bäckman]

Hi,

requesting reviews for this backport.

Webrev: http://cr.openjdk.java.net/~rbackman/8030976.8u40/
Bug: https://bugs.openjdk.java.net/browse/JDK-8030976
Commit: http://hg.openjdk.java.net/jdk9/hs/hotspot/rev/6ad207fd3e26

Thanks
/R