RFR: 8306075: Micro-optimize Enum.hashCode [v5]

[olivergillespie]

> Improve the speed of Enum.hashCode by caching the identity hashcode on first use. I've seen an application where Enum.hashCode is a hot path, and this is fairly simple speedup. The memory overhead is low; in enums with no extra fields there is already a 4-byte space due to alignment so this new field can slot in 'for free'. In other cases, the singleton nature of enum values means that the number of total instances is typically very low, so a small per-instance overhead is not a concern.
> 
> Please see more discussion/explanation in the [original enhancement request](https://bugs.openjdk.org/browse/JDK-8306075). [Benchmark results and generated code analysis moved to comment]
> 
> Thanks @shipilev for help with the implementation and interpreting the generated code.

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

  Switch to non-javadoc comment and remove markup

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Changes:
  - all: https://git.openjdk.org/jdk/pull/13491/files
  - new: https://git.openjdk.org/jdk/pull/13491/files/79af4952..f4b5c546

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13491&range=04
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13491&range=03-04

  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/13491.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13491/head:pull/13491

PR: https://git.openjdk.org/jdk/pull/13491