RFR: 8339198: Remove tag field from AbstractPoolEntry

[Claes Redestad]

On Thu, 19 Sep 2024 17:07:41 GMT, Chen Liang <liach at openjdk.org> wrote:

> The `tag` field in `AbstractPoolEntry` is redundant, as it's always of the same value for each subtype. This is now replaced with an override. This can reduce the footprint of the entries..
> 
> The removal of `hash` was considered but withdrawn; in part because hash computations are heavy; in c2 things as simple as identity hash code are better stored in object fields for performance.

Marked as reviewed by redestad (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/21093#pullrequestreview-2318678218