RFR: 8254354: Add an asExact() VarHandle combinator [v9]

[Jorn Vernee]

> Hi,
> 
> This patch adds an asExact() combinator to VarHandle, that will return a new VarHandle that performs exact type checks, similar to MethodHandle::invokeExact, to help developers catch inexact VarHandle usage, which can lead to performance degradation.
> 
> This is implemented using a boolean flag in VarForm. If the flag is set, the exact type of the invocation is checked against the exact type in the VarForm. If there is a mismatch, a WrongMethodTypeException is thrown.
> 
> Other than that, there is also an asGeneric() combinator added that does the inverse operation (thanks to Rémi for the suggestion). I've also added The `@Hidden` annotation to the VarHandleGuards methods, as well as a type-checking helper method called from the generic invocation lambda form, so that the stack trace we get points at the location where the VarHandle is being used.
> 
> Thanks,
> Jorn
> 
> CSR link: https://bugs.openjdk.java.net/browse/JDK-8255375

Jorn Vernee has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 12 additional commits since the last revision:

 - Update Javadoc, and rename asExact and asGeneric to withInvokeExactBehaviour and withInvokeBehaviour
 - Merge branch 'master' into Exact_VarHandle
 - Use AccessType ordinal in guard checks instead of the AccessMode ordinal
 - Update accessModeType to use the AccessType ordinal directly.
 - Add benchmarks
 - - Update javadoc
   - Make isExact() public
 - Fixes failing tests, and enable verifier on Exact test
 - Fix whitespace
 - Comment out VarHandleGuards generator code
 - Makes exactness a property of a VarHandle, not a VarForm, since the latter are shared. Use handle.accessModeType to get the exact type of the VarHandle.
 - ... and 2 more: https://git.openjdk.java.net/jdk/compare/f9c76729...3c707bc7

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/843/files
  - new: https://git.openjdk.java.net/jdk/pull/843/files/8e5fb451..3c707bc7

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=843&range=08
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=843&range=07-08

  Stats: 14492 lines in 455 files changed: 8478 ins; 4347 del; 1667 mod
  Patch: https://git.openjdk.java.net/jdk/pull/843.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/843/head:pull/843

PR: https://git.openjdk.java.net/jdk/pull/843