RFR: 8244813: [BACKOUT] 8244523: Shenandoah: Remove null-handling in LRB expansion

[Roman Kennke]

JDK-8244523 seems to cause a number of regression in nightlies (See:
JDK-8244663 and maybe JDK-8244721). Let's back it out until we figure
out the reasons for that.

The backout was not clean because of refactorings in the same code paths.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8244813
Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8244813/webrev.02/

Testing: hotspot_gc_shenandoah

Ok?

Roman