RFR: 8265079: Implement VarHandle invoker caching

[Jorn Vernee]

This patch implements 2 leftover TODOs for implementing var handle invoker MH caching (lambda forms for those were already shared/cached).

This piggybacks on the existing mechanism for method handle invoker caching.

Testing: Local testing `java/lang/invoke` tests. Tier 1-3

Thanks,
Jorn

-------------

Commit messages:
 - Add VarHandle invoker handle caching

Changes: https://git.openjdk.java.net/jdk/pull/3439/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3439&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8265079
  Stats: 115 lines in 2 files changed: 110 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3439.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3439/head:pull/3439

PR: https://git.openjdk.java.net/jdk/pull/3439