RFR: 8167353: [JVMCI] JVMCI re-initialization check is in the wrong location

[Doug Simon]

Please review this change that move a guard against JVMCI re-initialization to the right location. The chance of a race causing the check to fail when in the current/wrong location increased recently due to a change in Graal[1] which causes more JVMCI code to be run on application threads.

The webrev puts the check in the right place and also converts an assertion to a guarantee for a related check.

https://bugs.openjdk.java.net/browse/JDK-8167353
http://cr.openjdk.java.net/~dnsimon/8167353/

-Doug