RFR: 8244307: Improve assertions against taskqueue underflow

[Kim Barrett]

Please review this change to the assertions in the taskqueue code that
are checking for unexpected underflow of the buffer.  Temporary
underflow is possible in some situations, but it should always be
"quickly" resolved.  There were a number of different assertions of
this.  This change introduces an assert_not_underflow() operation,
giving them all a common name that makes the intent clearer.

CR:
https://bugs.openjdk.java.net/browse/JDK-8244307

Webrev:
https://cr.openjdk.java.net/~kbarrett/8244307/open.00/

Testing:
mach5 tier1-5, in combination with fix for JDK-8243325.
Locally (linux-x64) hotspot:tier1 without JDK-8243325.