RFR(XS): 8215044: C2 crash in loopTransform.cpp with assert(cl->trip_count() > 0) failed: peeling a fully unrolled loop

[Roland Westrelin]

http://cr.openjdk.java.net/~roland/8215044/webrev.00/

The trip count of the inner loop is first computed to be 4. The
pre/main/post loops are then created and the main loop trip count is
adjusted to be 3. The inner loop is unrolled once, trip count is set to
1 (3/2). Then peeling is attempted twice, the second one sees a zero
trip count.

The fix I propose is to not attempt peeling on a loop with an expected
trip count of 1.

Roland.