[jdk11u-dev] RFR: 8266018: Shenandoah: fix an incorrect assert

[Aleksey Shipilev]

Clean backport, passes `hotspot_gc_shenandoah`.

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Commit messages:
 - Backport 2dd56676fb564072f73f5fd9e591bda7922523ac

Changes: https://git.openjdk.java.net/jdk11u-dev/pull/67/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk11u-dev&pr=67&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266018
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk11u-dev/pull/67.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk11u-dev pull/67/head:pull/67

PR: https://git.openjdk.java.net/jdk11u-dev/pull/67