RFR: 8339198: Remove tag field from AbstractPoolEntry

[Chen Liang]

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The removal of `hash` was considered but withdrawn; in part because hash computations are heavy; in c2 things as simple as identity hash code are better stored in object fields for performance.

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Commit messages:
 - 8339198: Remove tag field from AbstractPoolEntry

Changes: https://git.openjdk.org/jdk/pull/21093/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21093&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8339198
  Stats: 140 lines in 1 file changed: 97 ins; 4 del; 39 mod
  Patch: https://git.openjdk.org/jdk/pull/21093.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21093/head:pull/21093

PR: https://git.openjdk.org/jdk/pull/21093