[11] RFR: 2020-04-15, Bulk backports to sh/jdk11

[Zhengyu Gu]

http://cr.openjdk.java.net/~zgu/sh-jdk11u-20200415/webrev.00/

Backport from jdk to shenandoah-jdk11u

[backport] 8237396: JvmtiTagMap::weak_oops_do() should not trigger
                     barriers
[backport] 8237632: Shenandoah: accept NULL fwdptr to cooperate with
                     JVMTI and JFR
[backport] 8240917: Shenandoah: Avoid scanning thread code roots twice
                     in all root scanner
[backport] 8241765: Shenandoah: AARCH64 need to save/restore call
                     clobbered registers before calling keepalive barrier
[backport] 8241900: Loop unswitching may cause dependence on null check
                     to be lost
[backport] 8242375: Shenandoah: Remove
                     ShenandoahHeuristic::record_gc_start/end methods

Test:
   hotspot_gc_shenandoah

Thanks,

-Zhengyu