RFR(S): 8244721: CTW: C2 (Shenandoah) compilation fails with "unexpected infinite loop graph shape"

[Roland Westrelin]

https://bugs.openjdk.java.net/browse/JDK-8244721
http://cr.openjdk.java.net/~roland/8244721/webrev.00/

Logic that finds raw memory in the case of an infinite loop fails if the
loop has more than one backedge. If there is no memory phi in the loop
head, that logic uses the memory input from the safepoint on the
backedge. With multiple backedges and multiple safepoints, all
safepoints should have the same memory input. I changed the logic so
it's robust to multiple backedges under that assumption.

Roland.