RFR(S): 8245083: [REDO] Shenandoah: Remove null-handling in LRB expansion

[Roland Westrelin]

https://bugs.openjdk.java.net/browse/JDK-8245083
http://cr.openjdk.java.net/~roland/8245083/webrev.00/

Same as 8244523, this removes null handling code from LRB expansion. The
only difference is that to preserve implicit null checks in:

a' = lrb(a);
if (a == null) { }
f = a'.f

this patch transforms it to:

a' = lrb(a);
if (a' == null) { }
f = a'.f

at expansion time.

Roland.