PhaseStringOpts crashes while optimising effectively dead code

The following test failed in the JDK15 CI:

tools/javac/annotations/typeAnnotations/classfile/CombinationsTargetTest3.java

Here's a snippet from the log file:

Test 30 parameters: tinv=0, tvis=4, inv=0, vis=0, Arepeats=true, BDrepeats=false, ABmix=false, retention: RUNTIME, anno2: TYPE,FIELD,METHOD,PARAMETER,CONSTRUCTOR,LOCAL_VARIABLE, src=src6
Testing classfile: Test30
MCODE: test, RuntimeVisibleTypeAnnotations: 4
  types:METHOD_REFERENCE
  types:METHOD_REFERENCE
  types:METHOD_REFERENCE_TYPE_ARGUMENT
  types:METHOD_REFERENCE_TYPE_ARGUMENT
Pass

Test 31 parameters: tinv=2, tvis=0, inv=2, vis=0, Arepeats=true, BDrepeats=false, ABmix=false, retention: CLASS, anno2: TYPE,FIELD,METHOD,PARAMETER,CONSTRUCTOR,LOCAL_VARIAB
----------stdout:(21/1020)*----------
#
# A fatal error has been detected by the Java Runtime Environment:
#
# EXCEPTION_ACCESS_VIOLATION (0xc0000005) at pc=0x00007ff83d58f1fd, pid=8788, tid=4020
#
# JRE version: Java(TM) SE Runtime Environment (15.0+28) (build 15-ea+28-1379)
# Java VM: Java HotSpot(TM) 64-Bit Server VM (15-ea+28-1379, mixed mode, tiered, compressed oops, g1 gc, windows-amd64)
# Problematic frame:
# V [jvm.dll+0x6bf1fd] PhaseStringOpts::replace_string_concat+0x12bd
#
# Core dump will be written. Default location: T:\\testoutput\\test-support\\jtreg_open_test_langtools_tier1\\scratch\\2\\hs_err_pid8788.mdmp
#
# An error report file with more information is saved as:
# T:\\testoutput\\test-support\\jtreg_open_test_langtools_tier1\\scratch\\2\\hs_err_pid8788.log
#
# Compiler replay data is saved as:
# T:\\testoutput\\test-support\\jtreg_open_test_langtools_tier1\\scratch\\2\\replay_pid8788.log
#
# If you would like to submit a bug report, please visit:
# https://bugreport.java.com/bugreport/crash.jsp
#
result: Error. Agent communication error: java.net.SocketException: Connection reset; check console log for any additional details


test result: Error. Agent communication error: java.net.SocketException: Connection reset; check console log for any additional details


Here's the crashing thread's stack:

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

Current thread (0x00000240b9513060): JavaThread "C2 CompilerThread2" daemon [_thread_in_native, id=4020, stack(0x000000b6a1400000,0x000000b6a1500000)]


Current CompileTask:
C2: 265338 18627 4 com.sun.tools.javac.main.Arguments::processArgs (46 bytes)

Stack: [0x000000b6a1400000,0x000000b6a1500000], sp=0x000000b6a14facc0, free space=1003k
Native frames: (J=compiled Java code, j=interpreted, Vv=VM code, C=native code)
V [jvm.dll+0x6bf1fd] PhaseStringOpts::replace_string_concat+0x12bd (stringopts.cpp:1832)
V [jvm.dll+0x6b96b0] PhaseStringOpts::PhaseStringOpts+0x4f0 (stringopts.cpp:667)
V [jvm.dll+0x1f1b56] Compile::inline_string_calls+0x106 (compile.cpp:1826)
V [jvm.dll+0x1f14d5] Compile::inline_incrementally+0x455 (compile.cpp:1956)
V [jvm.dll+0x1eb0e3] Compile::Optimize+0x243 (compile.cpp:2048)
V [jvm.dll+0x1e8e17] Compile::Compile+0xd17 (compile.cpp:738)
V [jvm.dll+0x177d97] C2Compiler::compile_method+0xc7 (c2compiler.cpp:115)
V [jvm.dll+0x1f8865] CompileBroker::invoke_compiler_on_method+0x765 (compileBroker.cpp:2223)
V [jvm.dll+0x1f690c] CompileBroker::compiler_thread_loop+0x28c (compileBroker.cpp:1900)
V [jvm.dll+0x742fad] JavaThread::run+0x12d (thread.cpp:1946)
V [jvm.dll+0x73bdba] Thread::call_run+0x8a (thread.cpp:403)
V [jvm.dll+0x628086] thread_native_entry+0xd6 (os_windows.cpp:466)
C [ucrtbase.dll+0x1f4a0]
C [KERNEL32.DLL+0x84d4]
C [ntdll.dll+0x6e8b1]


Since this is a Tier1 failure, I'm starting this bug off
as a P2 for initial triage.