RFR(XS): 8167300: Local scheduling failures during gcm should be fatal

[Roland Westrelin]

http://cr.openjdk.java.net/~roland/8167300/webrev.00/

As discussed in:
http://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/2016-September/024531.html

this change makes failure to schedule fatal in debug builds.

Roland.