RFR: 8260605: Various java.lang.invoke cleanups

[Claes Redestad]

- Remove unused code
- Inline and simplify the bootstrap method invocation code (remove pointless reboxing checks etc)
- Apply pattern matching to make some code more readable

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Commit messages:
 - Avoid unnecessary reboxing checks, inline and simplify class/mt invokes
 - j.l.invoke cleanups

Changes: https://git.openjdk.java.net/jdk/pull/2300/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2300&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260605
  Stats: 269 lines in 14 files changed: 11 ins; 161 del; 97 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2300.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2300/head:pull/2300

PR: https://git.openjdk.java.net/jdk/pull/2300