RFR/sh/jdk8): [backport] 8244663: Shenandoah: C2 assertion fails in Matcher::collect_null_checks

[Roman Kennke]

This is a backport of:

https://bugs.openjdk.java.net/browse/JDK-8244663
https://hg.openjdk.java.net/jdk/jdk/rev/1688221b62ab

JDK8 fix (thanks Roland for sorting it out):
http://cr.openjdk.java.net/~rkennke/8244663-jdk8/webrev.00/

Testing: hotspot_gc_shenandoah

Good?

Thanks,
Roman