RFR: JDK-8277520: Implement JDK-8 default methods for IdentityHashMap [v4]

[ExE Boss]

On Mon, 21 Feb 2022 23:36:19 GMT, liach <duke at openjdk.java.net> wrote:

>> Might need a CSR as now `computeIfAbsent` `computeIfPresent` `compute` `merge` would throw CME if the functions modified the map itself, and there are corresponding specification changes.
>
> liach has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - merge branch 'identityhashmap-bench' of https://github.com/liachmodded/jdk into identityhashmap-default
>  - fix whitespace

There should probably be something like [test/jdk/java/util/Collections/Wrappers.java](https://github.com/openjdk/jdk/blob/master/test/jdk/java/util/Collections/Wrappers.java) to check that `IdentityHashMap` overrides all `default` methods from `java.util.Map` (with `remove(K, V)` and `replace(K, V, V)` depending on <https://github.com/openjdk/jdk/pull/8259>).

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PR: https://git.openjdk.java.net/jdk/pull/6532