8339198: Remove tag field from AbstractPoolEntry

[liach]

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The removal of hash was considered but withdrawn; in part because hash computations are heavy; in c2 things as simple as identity hash code are better stored in object fields for performance.

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• JDK-8339198: Remove tag field from AbstractPoolEntry (Sub-task - P4)

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