[11u] Critical backports to sh/jdk11

[Aleksey Shipilev]

https://cr.openjdk.java.net/~shade/shenandoah/backports/jdk11-20190930/webrev.01/

This backports these critical fixes to sh/jdk11:
 [backport] 8231405: [Shenandoah] guarantee(d != NULL) failed: Null dominator info
 [backport] 8231583: Shenandoah: Fix register clash in SBSA::resolve_forwarding_pointer() borrowing

Both patches apply cleanly. I'll wait a bit for jdk/jdk nightlies to catch up with these.

Testing: hotspot_gc_shenandoah, tier1, tier2 (with Shenandoah), jdk_security (with Shenandoah),
selected benchmarks

-- 
Thanks,
-Aleksey