C2: assert(cnt == _outcnt) failed: no insertions allowed

The attached fuzzer test fails intermittently with the following assertion.

To reproduce:
$ java Test.java

#
# A fatal error has been detected by the Java Runtime Environment:
#
# Internal Error (/home/christian/jdk/open/src/hotspot/share/opto/node.cpp:214), pid=19747, tid=19759
# assert(cnt == _outcnt) failed: no insertions allowed
#
# JRE version: Java(TM) SE Runtime Environment (16.0) (slowdebug build 16-internal+0-2020-07-13-1111353.christian...)
# Java VM: Java HotSpot(TM) 64-Bit Server VM (slowdebug 16-internal+0-2020-07-13-1111353.christian..., mixed mode, tiered, compressed oops, g1 gc, linux-amd64)
# Problematic frame:
# V [libjvm.so+0xe5a558] DUIterator_Fast::verify(Node const*, bool)+0x58
..........
Command Line: --add-modules=ALL-DEFAULT jdk.compiler/com.sun.tools.javac.launcher.Main Test.java
..........
Current thread (0x00007f75f03537e0): JavaThread "C2 CompilerThread0" daemon [_thread_in_native, id=19759, stack(0x00007f75d80b4000,0x00007f75d81b5000)]


Current CompileTask:
C2: 8010 1788 4 Test::iMeth (221 bytes)

Stack: [0x00007f75d80b4000,0x00007f75d81b5000], sp=0x00007f75d81af050, free space=1004k
Native frames: (J=compiled Java code, A=aot compiled Java code, j=interpreted, Vv=VM code, C=native code)
V [libjvm.so+0xe5a558] DUIterator_Fast::verify(Node const*, bool)+0x58
V [libjvm.so+0x5e059e] DUIterator_Fast::operator++(int)+0x4a
V [libjvm.so+0xd00760] PhaseIdealLoop::insert_post_loop(IdealLoopTree*, Node_List&, CountedLoopNode*, CountedLoopEndNode*, Node*, Node*, CountedLoopNode*&)+0x864
V [libjvm.so+0xcfebd8] PhaseIdealLoop::insert_pre_post_loops(IdealLoopTree*, Node_List&, bool)+0x39a
V [libjvm.so+0xd07582] IdealLoopTree::iteration_split_impl(PhaseIdealLoop*, Node_List&)+0x388
V [libjvm.so+0xd07817] IdealLoopTree::iteration_split(PhaseIdealLoop*, Node_List&)+0x10b
V [libjvm.so+0xd07748] IdealLoopTree::iteration_split(PhaseIdealLoop*, Node_List&)+0x3c
V [libjvm.so+0xd07748] IdealLoopTree::iteration_split(PhaseIdealLoop*, Node_List&)+0x3c
V [libjvm.so+0xd078b5] IdealLoopTree::iteration_split(PhaseIdealLoop*, Node_List&)+0x1a9
V [libjvm.so+0xd078b5] IdealLoopTree::iteration_split(PhaseIdealLoop*, Node_List&)+0x1a9
V [libjvm.so+0xd1aae4] PhaseIdealLoop::build_and_optimize(LoopOptsMode)+0x1470
V [libjvm.so+0x6e1dfb] PhaseIdealLoop::PhaseIdealLoop(PhaseIterGVN&, LoopOptsMode)+0xb3
V [libjvm.so+0x6e1ef6] PhaseIdealLoop::optimize(PhaseIterGVN&, LoopOptsMode)+0x46
V [libjvm.so+0x6d64ff] Compile::Optimize()+0x907
V [libjvm.so+0x6cfaca] Compile::Compile(ciEnv*, ciMethod*, int, bool, bool, bool, bool, DirectiveSet*)+0x1090
V [libjvm.so+0x5ce7a7] C2Compiler::compile_method(ciEnv*, ciMethod*, int, bool, DirectiveSet*)+0x15b
V [libjvm.so+0x6ea926] CompileBroker::invoke_compiler_on_method(CompileTask*)+0x88e
V [libjvm.so+0x6e95b3] CompileBroker::compiler_thread_loop()+0x3df
V [libjvm.so+0x10a0bdf] compiler_thread_entry(JavaThread*, Thread*)+0x69
V [libjvm.so+0x109bd90] JavaThread::thread_main_inner()+0x146
V [libjvm.so+0x109bc41] JavaThread::run()+0x11f
V [libjvm.so+0x1097eca] Thread::call_run()+0x180
V [libjvm.so+0xea6716] thread_native_entry(Thread*)+0x1e4