RFR: 8256072: Eliminate JVMTI tagmap rehashing [v4]

[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.

Coleen Phillimore has updated the pull request incrementally with one additional commit since the last revision:

  Fix has_no_hash into fast_no_hash_check().

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Changes:
  - all: https://git.openjdk.org/jdk/pull/10938/files
  - new: https://git.openjdk.org/jdk/pull/10938/files/f214791d..8770d38c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10938&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10938&range=02-03

  Stats: 8 lines in 4 files changed: 3 ins; 1 del; 4 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