RFR: 8297784: Optimize @Stable field for Method.isCallerSensitive

[Aleksey Shipilev]

[JDK-8271820](https://bugs.openjdk.org/browse/JDK-8271820) introduced the `@Stable private Boolean callerSensitive;` cache in `Method`. That field is essentially a tri-state `Boolean` that relies on its `null` value to serve as "not initialized" value for `@Stable`.

This works well when the holder `Method` instance is constant: JIT compilers fold `@Stable` well. But if such folding fails, we always dereference the full-blown reference to "boxed" `Boolean` from the field on every access. We can instead do a more lean tri-state primitive field to improve that non-optimized case without sacrificing optimized case.

I chose `byte` and `-1`, `0`, `1` to let the fastpath encode well on most (all?) architectures.

On adhoc benchmark like:


@State(Scope.Thread)
public class WrapperConstness {
    static final Method CONST;
    static Method NON_CONST;

    static {
        try {
            CONST = WrapperConstness.class.getDeclaredMethod("target");
            NON_CONST = CONST;
        } catch (NoSuchMethodException e) {
            throw new RuntimeException(e);
        }
    }

    public void target() {}

    @Benchmark
    public void nonConstant() throws Exception {
        NON_CONST.invoke(this);
    }

    @Benchmark
    public void constant() throws Exception {
        CONST.invoke(this);
    }
}


We have a minor improvement for non-const case, confirmed due to better `isCallerSensitive` access:


Benchmark                     Mode  Cnt  Score   Error  Units

# Baseline
WrapperConstness.constant     avgt   25  0.410 ± 0.010  ns/op
WrapperConstness.nonConstant  avgt   25  7.283 ± 0.025  ns/op

# Patched
WrapperConstness.constant     avgt    5  0.407 ± 0.008  ns/op
WrapperConstness.nonConstant  avgt    5  7.054 ± 0.027  ns/op  ; -3%


Additional testing:
 - [x] Ad-hoc benchmarks
 - [x] Linux x86_64 fastdebug `java/lang/reflect`
 - [ ] Linux x86_64 fastdebug `tier1`, `tier2`

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

Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/11422/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11422&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297784
  Stats: 8 lines in 1 file changed: 3 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/11422.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11422/head:pull/11422

PR: https://git.openjdk.org/jdk/pull/11422