RFR: 8372845: C2: Fold identity hash code if object is constant [v6]

[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 incrementally with one additional commit since the last revision:

  Use UpperCamelCase

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Changes:
  - all: https://git.openjdk.org/jdk/pull/28589/files
  - new: https://git.openjdk.org/jdk/pull/28589/files/e33f01e0..77ec309b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28589&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28589&range=04-05

  Stats: 3 lines in 2 files changed: 0 ins; 0 del; 3 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