When doing the refactoring in JDK-8244509, I accidentally made a change that was not semantic-preserving: get_loop is called on new ctrl from IfTrueNode, not the ctrl we have been passed. It went correctly in test_null and test_in_cset, but not in test_gc_state.

Candidate fix:

diff -r 730eb308516a src/hotspot/share/gc/shenandoah/c2/shenandoahSupport.cpp
--- a/src/hotspot/share/gc/shenandoah/c2/shenandoahSupport.cpp Fri May 08 08:13:15 2020 -0400
+++ b/src/hotspot/share/gc/shenandoah/c2/shenandoahSupport.cpp Fri May 08 19:24:50 2020 +0200
@@ -877,11 +877,11 @@
 
   IfNode* gc_state_iff = new IfNode(old_ctrl, gc_state_bool, PROB_UNLIKELY(0.999), COUNT_UNKNOWN);
   ctrl = new IfTrueNode(gc_state_iff);
   test_fail_ctrl = new IfFalseNode(gc_state_iff);
 
- IdealLoopTree* loop = phase->get_loop(ctrl);
+ IdealLoopTree* loop = phase->get_loop(old_ctrl);
   phase->register_control(gc_state_iff, loop, old_ctrl);
   phase->register_control(ctrl, loop, gc_state_iff);
   phase->register_control(test_fail_ctrl, loop, gc_state_iff);
 
   phase->register_new_node(thread, old_ctrl);