RFR: 8303357: [JVMCI] thread is _thread_in_vm when committing JFR compilation event

[Doug Simon]

[JDK-8280844](https://bugs.openjdk.org/browse/JDK-8280844) added a native-to-VM thread transition when committing a JFR compilation event. This is not necessary if the thread is already `_thread_in_vm` at the commit point. This PR makes the transition conditional.

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

Commit messages:
 - only transition to _thread_in_vm if not already in that state

Changes: https://git.openjdk.org/jdk/pull/12787/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12787&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303357
  Stats: 10 lines in 1 file changed: 7 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/12787.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12787/head:pull/12787

PR: https://git.openjdk.org/jdk/pull/12787