The attached test was extracted from a newly added JCK test for 18.

To reproduce:

$ java -Xcomp -XX:-TieredCompilation -XX:CompileCommand=compileonly,Test::run -XX:CompileCommand=dontinline,Test::dontInline Test.java

#
# A fatal error has been detected by the Java Runtime Environment:
#
# Internal Error (/opt/mach5/mesos/work_dir/slaves/ff806ead-2cac-495d-9cbc-62116f99bf14-S13700/frameworks/1735e8a2-a1db-478c-8104-60c8b0af87dd-0196/executors/8f731315-8676-4d9a-b759-a859429b8bf4/runs/73843d3e-ae06-4fbd-97ab-7296fb2593fc/workspace/open/src/hotspot/share/opto/node.hpp:894), pid=20387, tid=20400
# assert(is_Loop()) failed: invalid node class: Region
#
# JRE version: Java(TM) SE Runtime Environment (18.0+26) (fastdebug build 18-ea+26-1787)
# Java VM: Java HotSpot(TM) 64-Bit Server VM (fastdebug 18-ea+26-1787, compiled mode, sharing, compressed oops, compressed class ptrs, g1 gc, linux-amd64)
# Problematic frame:
# V [libjvm.so+0x1352e34] IdealLoopTree::compute_trip_count(PhaseIdealLoop*)+0x34
.......
Command Line: -Xcomp -XX:-TieredCompilation -XX:CompileCommand=compileonly,Test::run -XX:CompileCommand=dontinline,Test::dontInline --add-modules=ALL-DEFAULT jdk.compiler/com.sun.tools.javac.launcher.Main Test.java
.......
Current CompileTask:
C2: 2416 81 b Test::run (29 bytes)

Stack: [0x00007fd3cc7d7000,0x00007fd3cc8d8000], sp=0x00007fd3cc8d1fb0, free space=1003k
Native frames: (J=compiled Java code, j=interpreted, Vv=VM code, C=native code)
V [libjvm.so+0x1352e34] IdealLoopTree::compute_trip_count(PhaseIdealLoop*)+0x34
V [libjvm.so+0x1366dec] IdealLoopTree::iteration_split(PhaseIdealLoop*, Node_List&)+0x11c
V [libjvm.so+0x13944c8] PhaseIdealLoop::build_and_optimize(LoopOptsMode)+0xe68
V [libjvm.so+0xa9485a] PhaseIdealLoop::optimize(PhaseIterGVN&, LoopOptsMode)+0x28a
V [libjvm.so+0xa9024a] Compile::Optimize()+0x73a
V [libjvm.so+0xa92d9e] Compile::Compile(ciEnv*, ciMethod*, int, Options, DirectiveSet*)+0x159e
V [libjvm.so+0x8ab5a4] C2Compiler::compile_method(ciEnv*, ciMethod*, int, bool, DirectiveSet*)+0x664
V [libjvm.so+0xaa3098] CompileBroker::invoke_compiler_on_method(CompileTask*)+0xc88
V [libjvm.so+0xaa3e58] CompileBroker::compiler_thread_loop()+0x648
V [libjvm.so+0x192745a] JavaThread::thread_main_inner()+0x25a
V [libjvm.so+0x192f820] Thread::call_run()+0x100
V [libjvm.so+0x1611f24] thread_native_entry(Thread*)+0x104