[8u] backport RFR(XS) 8068909: SIGSEGV in c2 compiled code with OptimizeStringConcat

[Vladimir Kozlov]

8u60 backport request.  Changes were pushed into jdk9 last week, no 
problems were found since then. Changes are applied to 8u cleanly.

https://bugs.openjdk.java.net/browse/JDK-8068909

review:
http://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/2015-January/016876.html
webrev:
http://cr.openjdk.java.net/~simonis/webrevs/2015/8068909.v2/
changeset
http://hg.openjdk.java.net/jdk9/hs-comp/hotspot/rev/8596a20c8de1

Thanks,
Vladimir