RFR: 8256072: Eliminate JVMTI tagmap rehashing

[Coleen Phillimore]

Use identity_hash for objects in the JVMTI TagMap table.  If the object has no hashcode, it's not in the table.
Tested with tier1-6.

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Commit messages:
 - 8256072: Eliminate JVMTI tagmap rehashing

Changes: https://git.openjdk.org/jdk/pull/10938/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10938&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8256072
  Stats: 108 lines in 12 files changed: 10 ins; 93 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/10938.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10938/head:pull/10938

PR: https://git.openjdk.org/jdk/pull/10938