RFR: 8284579: Improve VarHandle checks for interpreter

[Claes Redestad]

A few additional enhancements aiming to improve VH performance in the interpreter:

- Flatten `TypeAndInvokers`: adds a pointer to `VarHandle` (a small increase 40->48) but removes an object and an indirection on any instance actually used - and might avoid allocating the `MethodHandle[]` unnecessarily on some instances
- Have `checkExactAccessMode` return the directness of the `VarHandle` so that we can avoid some `isDirect` method calls.

Baseline

Benchmark                                 Mode  Cnt    Score   Error  Units
VarHandleExact.exact_exactInvocation      avgt   30  478.324 ? 5.762  ns/op
VarHandleExact.generic_exactInvocation    avgt   30  392.114 ? 1.644  ns/op
VarHandleExact.generic_genericInvocation  avgt   30  822.484 ? 1.865  ns/op


Patched

Benchmark                                 Mode  Cnt    Score   Error  Units
VarHandleExact.exact_exactInvocation      avgt   30  437.704 ? 5.320  ns/op
VarHandleExact.generic_exactInvocation    avgt   30  374.512 ? 3.154  ns/op
VarHandleExact.generic_genericInvocation  avgt   30  757.054 ? 1.237  ns/op

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Commit messages:
 - Various improvements to VarHandle startup

Changes: https://git.openjdk.java.net/jdk/pull/8160/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8160&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8284579
  Stats: 232 lines in 4 files changed: 18 ins; 23 del; 191 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8160.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8160/head:pull/8160

PR: https://git.openjdk.java.net/jdk/pull/8160