RFR: Inline usage of pointer_delta

[William Kemper]

Card table sometimes erroneously triggers assert added in JDK-8260046.

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Commit messages:
 - Inline usage of pointer_delta

Changes: https://git.openjdk.java.net/shenandoah/pull/83/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=83&range=00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/83.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/83/head:pull/83

PR: https://git.openjdk.java.net/shenandoah/pull/83