RFR(XS): 8247763: assert(outer->outcnt() == 2) failed: 'only phis' failure in LoopNode::verify_strip_mined()

[Roland Westrelin]

https://bugs.openjdk.java.net/browse/JDK-8247763
http://cr.openjdk.java.net/~roland/8247763/webrev.00/

A store is sunk from a pre loop and control is set to the outer strip
mined loop of the main loop (the only use of the store is the phi of the
main loop). Fix simply sets control to entry control in that case.

Roland.