Shenandoah: C2 failures after JDK-8255000

Summary is provisionary, adjust when finding more details.

Many tests exhibit the following assert with Shenandoah:

# Internal Error (/home/rkennke/src/openjdk/jdk/src/hotspot/share/gc/shenandoah/c2/shenandoahSupport.cpp:1909), pid=358095, tid=358108
# Error: assert(controls.member(u->in(0))) failed

See also attached hs_err and replay files.

Reproduced by (for example):
make run-test TEST=vmTestbase/nsk/jdi/ThreadReference/forceEarlyReturn/forceEarlyReturn006/TestDescription.java TEST_VM_OPTS="-XX:+UseShenandoahGC"

Bisecting shows that this regression is caused by JDK-8255000.