RFR: 8289003: JavaThread::check_is_terminated() implementation should rely on Thread-SMR

[Daniel D. Daugherty]

A small fix to change `JavaThread::check_is_terminated()` to rely on Thread-SMR.
The change is textually trivial, but requires a bit of thinking about the JavaThread
lifecycle. Since this is a cleanup of some old Thread-SMR related changes, I also
took the time to cleanup some comments and white space while I was visiting the
uses of `_vm_exited`.

Mach5 Tier1 has no related failures. Mach5 Tier[23] testing has just started.

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Commit messages:
 - 8289003: JavaThread::check_is_terminated() implementation should rely on Thread-SMR

Changes: https://git.openjdk.org/jdk/pull/9502/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9502&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8289003
  Stats: 10 lines in 3 files changed: 1 ins; 2 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/9502.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9502/head:pull/9502

PR: https://git.openjdk.org/jdk/pull/9502