RFR: 8372845: Fold identity hash code if object is constant

[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.

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Commit messages:
 - Move around
 - Constant fold identity hash

Changes: https://git.openjdk.org/jdk/pull/28589/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28589&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8372845
  Stats: 88 lines in 4 files changed: 87 ins; 0 del; 1 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