RFR: 8292910: Remove root to oop cast in HeapDumper

[Stefan Karlsson]

Small cleanup to fix an unclean cast. This patch was brought over from the Generational ZGC repository, where we have extra verification code around casts to oops.

I deduplicated the pointer dumping implementation, instead of adding a third version looking more or less the same.

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Commit messages:
 - 8292910: Remove root to oop cast in HeapDumper

Changes: https://git.openjdk.org/jdk/pull/10018/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10018&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8292910
  Stats: 21 lines in 1 file changed: 12 ins; 6 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/10018.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10018/head:pull/10018

PR: https://git.openjdk.org/jdk/pull/10018