RFR(S): 8247824: CTW: C2 (Shenandoah) compilation fails with SEGV in SBC2Support::pin_and_expand

[Roland Westrelin]

https://bugs.openjdk.java.net/browse/JDK-8247824
http://cr.openjdk.java.net/~roland/8247824/webrev.00/

If a barrier is expanded in the outer loop of a strip mined loop nest,
the outer loop head is changed to a new LoopNode so loop strip mining
verification code doesn't trigger and fail. The crash occurs when
there's 2 barriers in the outer loop and C2 attempts to transform the
loop head twice. The second time, loop->_head points to a dead node.

Roland.