[jdk11u-dev] RFR: 8266813: Shenandoah: Use shorter instruction sequence for checking if marking in progress

[Aleksey Shipilev]

Clean backport. Passes `hotspot_gc_shenandoah` on AArch64.

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Commit messages:
 - Backport 9c9c47e403c6d38afd1ce89f46a1887a01964796

Changes: https://git.openjdk.java.net/jdk11u-dev/pull/68/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk11u-dev&pr=68&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266813
  Stats: 3 lines in 1 file changed: 0 ins; 2 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk11u-dev/pull/68.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk11u-dev pull/68/head:pull/68

PR: https://git.openjdk.java.net/jdk11u-dev/pull/68