RFR: 8278341: Liveness check for global scope is not as fast as it could be

[Maurizio Cimadamore]

When doing some unrelated performance measurements, I realized that segments backed by global scope were still paying a relatively high cost for liveness checks - that's because GlobalScopeImpl extends from SharedScopeImpl, and does not override the `isAlive` method. This means that when checking for liveness, we will still do (in some cases - e.g. when calling `checkValidStateSlow`) a volatile VH get on the scope state - which is useless in this case.

This simple patch adds the missing override.

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Commit messages:
 - Override isAlive in global scope implementation

Changes: https://git.openjdk.java.net/jdk/pull/6744/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6744&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278341
  Stats: 5 lines in 1 file changed: 5 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6744.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6744/head:pull/6744

PR: https://git.openjdk.java.net/jdk/pull/6744