I found this while trying to reproduce another bug:

# A fatal error has been detected by the Java Runtime Environment:
#
# Internal Error (/oracle/jdk/open/src/hotspot/share/opto/node.hpp:886), pid=183049, tid=183062
# assert(is_Loop()) failed: invalid node class: Region
#
# JRE version: Java(TM) SE Runtime Environment (18.0) (fastdebug build 18-internal+0-2021-07-08-1412541.tobias...)
# Java VM: Java HotSpot(TM) 64-Bit Server VM (fastdebug 18-internal+0-2021-07-08-1412541.tobias..., mixed mode, tiered, compressed oops, compressed class ptrs, g1 gc, linux-amd64)
# Problematic frame:
# V [libjvm.so+0x12b5f14] IdealLoopTree::compute_trip_count(PhaseIdealLoop*)+0x34
#
# Core dump will be written. Default location: Core dumps may be processed with "/usr/share/apport/apport %p %s %c %d %P %E" (or dumping to /oracle/jdk/open/core.183049)
#
# If you would like to submit a bug report, please visit:
# https://bugreport.java.com/bugreport/crash.jsp
#

--------------- S U M M A R Y ------------

Command Line: -XX:CompileCommand=dontinline,Test::method -XX:CompileCommand=quiet -XX:CompileCommand=compileonly,Test::test Test

Host: prometheus, Intel(R) Core(TM) i7-9850H CPU @ 2.60GHz, 12 cores, 31G, Ubuntu 20.04.2 LTS
Time: Thu Jul 15 10:29:33 2021 CEST elapsed time: 0.395168 seconds (0d 0h 0m 0s)

--------------- T H R E A D ---------------

Current thread (0x00007f4a6c2e31d0): JavaThread "C2 CompilerThread0" daemon [_thread_in_native, id=183062, stack(0x00007f4a32f39000,0x00007f4a3303a000)]


Current CompileTask:
C2: 395 26 % ! 4 Test::test @ 20 (57 bytes)

Stack: [0x00007f4a32f39000,0x00007f4a3303a000], sp=0x00007f4a33034340, free space=1004k
Native frames: (J=compiled Java code, j=interpreted, Vv=VM code, C=native code)
V [libjvm.so+0x12b5f14] IdealLoopTree::compute_trip_count(PhaseIdealLoop*)+0x34
V [libjvm.so+0x12ca73e] IdealLoopTree::iteration_split(PhaseIdealLoop*, Node_List&)+0xfe
V [libjvm.so+0x12f6c98] PhaseIdealLoop::build_and_optimize(LoopOptsMode)+0xd58
V [libjvm.so+0xa10cca] PhaseIdealLoop::optimize(PhaseIterGVN&, LoopOptsMode)+0x2da
V [libjvm.so+0xa0c676] Compile::Optimize()+0x586
V [libjvm.so+0xa0f168] Compile::Compile(ciEnv*, ciMethod*, int, bool, bool, bool, bool, bool, DirectiveSet*)+0x1618
V [libjvm.so+0x82c316] C2Compiler::compile_method(ciEnv*, ciMethod*, int, bool, DirectiveSet*)+0x646
V [libjvm.so+0xa1fbe9] CompileBroker::invoke_compiler_on_method(CompileTask*)+0xef9
V [libjvm.so+0xa20918] CompileBroker::compiler_thread_loop()+0x598
V [libjvm.so+0x188af5c] JavaThread::thread_main_inner()+0x27c
V [libjvm.so+0x1891700] Thread::call_run()+0x100
V [libjvm.so+0x156f284] thread_native_entry(Thread*)+0x104