RFR: 8284579: Improve VarHandle checks for interpreter [v4]

[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, `-Xint`
> 
> 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, `-Xint`
> 
> 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
> 
> 
> No significant performance difference in normal mode.

Claes Redestad has updated the pull request incrementally with two additional commits since the last revision:

 - Improve javadoc for merged method
 - Add javadoc for merged method

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8160/files
  - new: https://git.openjdk.java.net/jdk/pull/8160/files/2a4fbd6d..df1d652b

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8160&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8160&range=02-03

  Stats: 10 lines in 1 file changed: 10 ins; 0 del; 0 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