RFR: 8372845: C2: Fold identity hash code if object is constant [v4]

[Chen Liang]

> Folding identity hash as constant if the incoming argument is constant would be useful for quick map lookups, such as for the [Classifier proposal](https://openjdk.org/jeps/8357674). Currently, identity hash is not constant because it loads the object header/mark word. We can add an explicit bypass to load an existing hash eagerly instead.

Chen Liang 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 12 additional commits since the last revision:

 - Move test, fix merge garbage
 - Merge branch 'master' of https://github.com/openjdk/jdk into fix/identity-hash-const
 - Merge branch 'master' of https://github.com/openjdk/jdk into fix/identity-hash-const
 - Typo
 - assert
 - refactorings
 - Typo
 - Merge branch 'master' of https://github.com/openjdk/jdk into fix/identity-hash-const
 - Cleanup
 - identity hash support in C2
 - ... and 2 more: https://git.openjdk.org/jdk/compare/9543b2fa...67a3954f

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/28589/files
  - new: https://git.openjdk.org/jdk/pull/28589/files/b1d8be39..67a3954f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28589&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28589&range=02-03

  Stats: 44910 lines in 756 files changed: 29623 ins; 11706 del; 3581 mod
  Patch: https://git.openjdk.org/jdk/pull/28589.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28589/head:pull/28589

PR: https://git.openjdk.org/jdk/pull/28589