(synopsis is provisional, please change as you see fit)

Found by fuzzing. Crashes every time with the generated test. Test bundle is attached, it also includes hs_errs.

$ java Test
Default case invoked for:
   opcode = 102, "Con"
o1 Con === o0 [[]] #top

--N: o1 Con === o0 [[]] #top

# Internal Error (/home/shade/trunks/jdk-jdk/src/hotspot/share/opto/matcher.cpp:1588), pid=26509, tid=26566
# assert(false) failed: bad AD file
#
# JRE version: OpenJDK Runtime Environment (14.0) (fastdebug build 14-internal+0-adhoc.shade.jdk-jdk)
# Java VM: OpenJDK 64-Bit Server VM (fastdebug 14-internal+0-adhoc.shade.jdk-jdk, mixed mode, sharing, tiered, compressed oops, g1 gc, linux-amd64)
# Problematic frame:
# V [libjvm.so+0x12c50dd] Matcher::Label_Root(Node const*, State*, Node*, Node const*)+0x51d

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

Current thread (0x00007f21bc009000): JavaThread "C2 CompilerThread1" daemon [_thread_in_native, id=26566, stack(0x00007f21eda02000,0x00007f21edb03000)]

Current CompileTask:
C2: 749 76 ! 4 Test::vMeth (1622 bytes)

Stack: [0x00007f21eda02000,0x00007f21edb03000], sp=0x00007f21edafdac0, free space=1006k
Native frames: (J=compiled Java code, A=aot compiled Java code, j=interpreted, Vv=VM code, C=native code)
V [libjvm.so+0x12c50dd] Matcher::Label_Root(Node const*, State*, Node*, Node const*)+0x51d
V [libjvm.so+0x12c5005] Matcher::Label_Root(Node const*, State*, Node*, Node const*)+0x445
V [libjvm.so+0x12c5445] Matcher::match_tree(Node const*)+0x1e5
V [libjvm.so+0x12c85ea] Matcher::xform(Node*, int)+0x9ba
V [libjvm.so+0x12cb917] Matcher::match()+0xb07
V [libjvm.so+0x9c0c15] Compile::Code_Gen()+0x85
V [libjvm.so+0x9c5294] Compile::Compile(ciEnv*, C2Compiler*, ciMethod*, int, bool, bool, bool, DirectiveSet*)+0x1244
V [libjvm.so+0x8004cd] C2Compiler::compile_method(ciEnv*, ciMethod*, int, DirectiveSet*)+0x10d
V [libjvm.so+0x9d1767] CompileBroker::invoke_compiler_on_method(CompileTask*)+0x397
V [libjvm.so+0x9d28d8] CompileBroker::compiler_thread_loop()+0x4f8
V [libjvm.so+0x182ce1a] JavaThread::thread_main_inner()+0x26a
V [libjvm.so+0x1834a08] JavaThread::run()+0x238
V [libjvm.so+0x1832456] Thread::call_run()+0xf6
V [libjvm.so+0x143045e] thread_native_entry(Thread*)+0x10e