Various crashes in C2's register allocator

The following test failed in the JDK15 CI:

gc/g1/humongousObjects/TestHumongousNonArrayAllocation.java

Here's a snippet from the log file:

#section:main
----------messages:(5/655)----------
command: main -Xms128M -Xmx128M -XX:+UseG1GC -XX:+UnlockDiagnosticVMOptions -XX:+WhiteBoxAPI -Xbootclasspath/a:. -XX:G1HeapRegionSize=1M gc.g1.humongousObjects.TestHumongousNonArrayAllocation TWO_REGION_HUMONGOUS
reason: User specified action: run main/othervm -Xms128M -Xmx128M -XX:+UseG1GC -XX:+UnlockDiagnosticVMOptions -XX:+WhiteBoxAPI -Xbootclasspath/a:. -XX:G1HeapRegionSize=1M gc.g1.humongousObjects.TestHumongousNonArrayAllocation TWO_REGION_HUMONGOUS
Mode: othervm [/othervm specified]
Additional options from @modules: --add-modules java.base,java.management --add-exports java.base/jdk.internal.misc=ALL-UNNAMED
elapsed time (seconds): 18.546
----------configuration:(4/111)----------
Boot Layer
  add modules: java.base java.management
  add exports: java.base/jdk.internal.misc ALL-UNNAMED

----------System.out:(27/1542)----------
[2020-03-06T10:08:08.526665Z] Gathering output for process 54514
[2020-03-06T10:08:08.868181Z] Waiting for completion for process 54514
#
# A fatal error has been detected by the Java Runtime Environment:
#
# SIGSEGV (0xb) at pc=0x000000010987a406, pid=54512, tid=24579
#
# JRE version: Java(TM) SE Runtime Environment (15.0+14) (fastdebug build 15-ea+14-506)
# Java VM: Java HotSpot(TM) 64-Bit Server VM (fastdebug 15-ea+14-506, mixed mode, tiered, compressed oops, g1 gc, bsd-amd64)
# Problematic frame:
# V [libjvm.dylib+0x27a406] IndexSet::insert(unsigned int)+0x3e
#
# Core dump will be written. Default location: core.54512
#
Unsupported internal testing APIs have been used.

# An error report file with more information is saved as:
# /mesos/work_dir/slaves/7cfa21b9-afde-4338-a459-d3e0a13ec793-S1176/frameworks/1735e8a2-a1db-478c-8104-60c8b0af87dd-0196/executors/a3ef3fac-36fb-4b84-834b-698120fc83be/runs/f2e22799-72c5-450d-b4ab-af4a6651a728/testoutput/test-support/jtreg_open_test_hotspot_jtreg_tier1_gc/scratch/0/hs_err_pid54512.log
#
# Compiler replay data is saved as:
# /mesos/work_dir/slaves/7cfa21b9-afde-4338-a459-d3e0a13ec793-S1176/frameworks/1735e8a2-a1db-478c-8104-60c8b0af87dd-0196/executors/a3ef3fac-36fb-4b84-834b-698120fc83be/runs/f2e22799-72c5-450d-b4ab-af4a6651a728/testoutput/test-support/jtreg_open_test_hotspot_jtreg_tier1_gc/scratch/0/replay_pid54512.log
#
# If you would like to submit a bug report, please visit:
# https://bugreport.java.com/bugreport/crash.jsp
#
Current thread is 24579
Dumping core ...
----------System.err:(3/172)----------
java version "15-ea" 2020-09-15
Java(TM) SE Runtime Environment (fastdebug build 15-ea+14-506)
Java HotSpot(TM) 64-Bit Server VM (fastdebug build 15-ea+14-506, mixed mode)
----------rerun:(43/7121)*----------


Here's the crashing thread's stack:

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

Current thread (0x00007ff58a800800): JavaThread "C2 CompilerThread1" daemon [_thread_in_native, id=24579, stack(0x000070000916b000,0x000070000926b000)]


Current CompileTask:
C2: 12508 401 4 java.util.regex.Pattern$GroupHead::match (47 bytes)

Stack: [0x000070000916b000,0x000070000926b000], sp=0x0000700009267150, free space=1008k
Native frames: (J=compiled Java code, A=aot compiled Java code, j=interpreted, Vv=VM code, C=native code)
V [libjvm.dylib+0x27a406] IndexSet::insert(unsigned int)+0x3e
V [libjvm.dylib+0x4f6c92] PhaseIFG::SquareUp()+0x80
V [libjvm.dylib+0x276176] PhaseChaitin::Register_Allocate()+0x768
V [libjvm.dylib+0x304280] Compile::Code_Gen()+0x2ba
V [libjvm.dylib+0x30201d] Compile::Compile(ciEnv*, C2Compiler*, ciMethod*, int, bool, bool, bool, DirectiveSet*)+0xbf1
V [libjvm.dylib+0x254096] C2Compiler::compile_method(ciEnv*, ciMethod*, int, DirectiveSet*)+0xe8
V [libjvm.dylib+0x3141fb] CompileBroker::invoke_compiler_on_method(CompileTask*)+0x555
V [libjvm.dylib+0x313a9b] CompileBroker::compiler_thread_loop()+0x283
V [libjvm.dylib+0xabb521] JavaThread::thread_main_inner()+0x193
V [libjvm.dylib+0xab7cfb] Thread::call_run()+0x11b
V [libjvm.dylib+0x931694] thread_native_entry(Thread*)+0xe0
C [libsystem_pthread.dylib+0x3661] _pthread_body+0x154
C [libsystem_pthread.dylib+0x350d] _pthread_body+0x0
C [libsystem_pthread.dylib+0x2bf9] thread_start+0xd


siginfo: si_signo: 11 (SIGSEGV), si_code: 0 (unknown), si_addr: 0x0000000000000000