RFR: (sh/jdk11): [backport] 8228775: Shenandoah: Remove useless null-input-verification in Shenandoah/C2 verifier

[Roman Kennke]

Let's backport this real quick to unbreak nightlies.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8228775
Webrev (transplant applied cleanly, will add [backport] in msg):
http://cr.openjdk.java.net/~rkennke/JDK-8228775/webrev.00/

Testing: hotspot_gc_shenandoah, failing
specjvm/+ShenandoahVerifyOptoBarriers test

Ok?

Roman