RFR: Backport reshaped write barrier expansion to JDK11

[Roman Kennke]

A while ago, we re-shaped our WB expansion in sh/jdk. This backports the
new shape to sh/jdk11.

http://cr.openjdk.java.net/~rkennke/shjdk11-wb-expansion/webrev.00/

Roland: please carefully check this for sanity's sake

I intend to push this *after* we tagged the tree. This C2 backporting
stuff may become a little rocky.

Testing: hotspot_gc_shenandoah (fastdebug+release)

Roman