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

Found with fuzzing. Reliably reproduces on aarch64, but not on x86_64, although it is probably not architecture-specific.
Test bundle is attached, along with sample hs_errs.

$ ~/jdk-jdk/build/linux-aarch64-server-fastdebug/images/jdk/bin/java Test

# A fatal error has been detected by the Java Runtime Environment:
#
#  Internal Error (/home/shade/jdk-jdk/src/hotspot/share/opto/compile.cpp:3489), pid=3503, tid=3514
#  assert(!wq.contains(u)) failed: shouldn't process one node several times
#
# 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-aarch64)
# Problematic frame:
# V  [libjvm.so+0x7a0f58]  Compile::final_graph_reshaping_main_switch(Node*, Final_Reshape_Counts&, unsigned int)+0x1268
#
# Core dump will be written. Default location: Core dumps may be processed with "/usr/libexec/abrt-hook-ccpp %s %c %p %u %g %t e %P %I %h" (or dumping to /home/shade/tests/05015/core.3503)
#
# If you would like to submit a bug report, please visit:
#   http://bugreport.java.com/bugreport/crash.jsp
#

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

Command Line: Test

Host: merino.lab2.eng.bos.redhat.com, AArch64, 46 cores, 95G, Red Hat Enterprise Linux Server release 7.6 (Maipo)
Time: Wed Aug 14 06:39:26 2019 EDT elapsed time: 1 seconds (0d 0h 0m 1s)

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

Current thread (0x0000ffff905ab000):  JavaThread "C2 CompilerThread0" daemon [_thread_in_native, id=3514, stack(0x0000ffff2cf90000,0x0000ffff2d190000)]


Current CompileTask:
C2:   1335   88 % !   4       Test::vMeth @ 72 (368 bytes)

Stack: [0x0000ffff2cf90000,0x0000ffff2d190000],  sp=0x0000ffff2d18aae0,  free space=2026k
Native frames: (J=compiled Java code, A=aot compiled Java code, j=interpreted, Vv=VM code, C=native code)
V  [libjvm.so+0x7a0f58]  Compile::final_graph_reshaping_main_switch(Node*, Final_Reshape_Counts&, unsigned int)+0x1268
V  [libjvm.so+0x7a1d04]  Compile::final_graph_reshaping_impl(Node*, Final_Reshape_Counts&)+0x1a4
V  [libjvm.so+0x7a227c]  Compile::final_graph_reshaping_walk(Node_Stack&, Node*, Final_Reshape_Counts&)+0xcc
V  [libjvm.so+0x7abc40]  Compile::final_graph_reshaping()+0x3f8
V  [libjvm.so+0x7adee4]  Compile::Optimize()+0xdfc
V  [libjvm.so+0x7af014]  Compile::Compile(ciEnv*, C2Compiler*, ciMethod*, int, bool, bool, bool, DirectiveSet*)+0x10a0
V  [libjvm.so+0x61d428]  C2Compiler::compile_method(ciEnv*, ciMethod*, int, DirectiveSet*)+0x114