[8u] RFR + RFA: 8209639: assert failure in coalesce.cpp: attempted to spill a non-spillable item

[Roland Westrelin]

I'd like to backport 8209639 to 8u because one of our customers is
hitting it. The change doesn't apply cleanly so here is a webrev for 8u:

http://cr.openjdk.java.net/~roland/8209639-8u/webrev.00/

Initial change:

https://bugs.openjdk.java.net/browse/JDK-8209639
http://hg.openjdk.java.net/jdk/jdk/rev/76a51e26d0ac

Roland.