RFR: 8351087: Combine scratch object tables in heapShared.cpp

[Ioi Lam]

Please review this trivial change. The two tables `_scratch_java_mirror_table` and `_scratch_references_table` are always disjoint. So we can simply combine them into a single table.

I am running tiers 1-5 for sanity sake.

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Commit messages:
 - 8351087: Combine scratch object tables in heapShared.cpp

Changes: https://git.openjdk.org/jdk/pull/23876/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23876&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8351087
  Stats: 16 lines in 3 files changed: 0 ins; 3 del; 13 mod
  Patch: https://git.openjdk.org/jdk/pull/23876.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23876/head:pull/23876

PR: https://git.openjdk.org/jdk/pull/23876