RFR: 8243325: Cleanup TaskQueueSuper<>::peek

[Kim Barrett]

Please review this change to some assertions in taskTerminator that
are verifying all the queues are empty.  Previously this was
accomplished by using the peek operation on the queues, verifying all
returned false.  That's the only use of the peek operation.  This
change replaces that usage with new assert_empty helper functions, and
eliminates the now unused peek operations.

CR:
https://bugs.openjdk.java.net/browse/JDK-8243325

Webrev:
https://cr.openjdk.java.net/~kbarrett/8243325/open.00/

Testing:
mach5 tier1-5, in combination with fix for JDK-8244307.
Locally (linux-x64) hotspot:tier1 without JDK-8244307.