--- a/src/hotspot/share/c1/c1_GraphBuilder.cpp	Fri Nov 08 11:15:16 2019 +0000
+++ b/src/hotspot/share/c1/c1_GraphBuilder.cpp	Tue Nov 12 16:13:16 2019 +0100
@@ -2590,7 +2590,7 @@
 
 #ifdef ASSERT
   for_each_phi_fun(b, phi,
-                   assert(phi->operand_count() != 1 || phi->subst() != phi, "missed trivial simplification");
+                   assert(phi->operand_count() != 1 || phi->subst() != phi || phi->is_illegal(), "missed trivial simplification");
   );
 
   ValueStack* state = b->state()->caller_state();