RFR: 8320418: PPC64: invokevfinal_helper duplicates code to handle ResolvedMethodEntry

[Martin Doerr]

`TemplateTable::invokevfinal_helper` should use `TemplateTable::prepare_invoke`. `TemplateInterpreter::invoke_return_entry_table_for` needs to support `_fast_invokevfinal` bytecode for that which is only used by PPC64. (It is probably still beneficial for AIX which doesn't support CDS.)
In addition, I've cleaned up some inaccurate comments.

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Commit messages:
 - 8320418: PPC64: invokevfinal_helper duplicates code to handle ResolvedMethodEntry

Changes: https://git.openjdk.org/jdk/pull/16741/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16741&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8320418
  Stats: 38 lines in 3 files changed: 5 ins; 16 del; 17 mod
  Patch: https://git.openjdk.org/jdk/pull/16741.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16741/head:pull/16741

PR: https://git.openjdk.org/jdk/pull/16741