RFR: 8256072: Eliminate JVMTI tagmap rehashing [v2]

[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 with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains two additional commits since the last revision:

 - Merge branch 'master' into jvmti
 - 8256072: Eliminate JVMTI tagmap rehashing

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/10938/files
  - new: https://git.openjdk.org/jdk/pull/10938/files/29fb0c2f..e549dcb5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10938&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10938&range=00-01

  Stats: 32521 lines in 114 files changed: 2966 ins; 29135 del; 420 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