RFR(S): 8217042: Shenandoah: write barrier on backedge of strip mined loop causes c2 crash at expansion time

[Roland Westrelin]

http://cr.openjdk.java.net/~roland/8217042/webrev.00/

If a write barrier is in the body of the outer strip mined loop,
expanding it causes loop strip mining verification code to fail. This is
worked around by turning the strip mined loop nest into a regular
counted loop nest so verification code doesn't trigger. The logic that
takes care of that breaks when the write barrier is on the backedge of
the strip mined loop because it is applied after the barrier is
expanded. The fix I propose is to move that logic before barrier
expansion.

Roland.