RFR(S): 8231405: [Shenandoah] guarantee(d != NULL) failed: Null dominator info

[Roland Westrelin]

http://cr.openjdk.java.net/~roland/8231405/webrev.00/

A load barrier can sometimes be scheduled right after a call to the
rethrow stub. The rethrow stub has more projection than a usual call and
the logic to duplicate load barriers along the exception/fallthrough
paths can't work. There can't be any dependency between the load barrier
and the rethrow stub so the fix moves the barrier above the call.

Roland.