RFR(S): 8186125: "DU iteration must converge quickly" assert in split if with unsafe accesses

[Roland Westrelin]

http://cr.openjdk.java.net/~roland/8186125/webrev.00/

Split if is missing support for graph shapes with the Opaque4Node that
was introduced for unsafe accesses by JDK-8176506.

In the test case, the 2 Unsafe accesses share a single Opaque4Node
before the if. When split if encounters the Cmp->Bol->Opaque4->If chain,
it only tries to clone Cmp->Bol when it should clone Cmp->Bol->Opaque4
to make one copy for each If.

Roland.