G1 asserts nmethod should not be unloaded during parallel code cache unloading

Test
compiler/jsr292/NonInlinedCall/GCTest.java
failed with
#
# A fatal error has been detected by the Java Runtime Environment:
#
# Internal Error (/scratch/opt/mach5/mesos/work_dir/slaves/2dd962d0-8988-479b-a804-57ab764ada59-S1201/frameworks/1735e8a2-a1db-478c-8104-60c8b0af87dd-0196/executors/65e1fcaf-db47-4be1-9994-485861753ee6/runs/2ec665b1-7ab6-4c49-afcc-f825629942e5/workspace/open/src/hotspot/share/code/nmethod.cpp:1683), pid=4018, tid=4025
# assert(!is_unloaded()) failed: should not call follow on unloaded nmethod
#
# JRE version: Java(TM) SE Runtime Environment (13.0+9) (fastdebug build 13-ea+9)
# Java VM: Java HotSpot(TM) 64-Bit Server VM (fastdebug 13-ea+9, mixed mode, compressed oops, g1 gc, linux-amd64)
# Problematic frame:
# V [libjvm.so+0x12d5bf8] nmethod::oops_do(OopClosure*, bool)+0x388
#
# Core dump will be written. Default location: /scratch/opt/mach5/mesos/work_dir/slaves/2dd962d0-8988-479b-a804-57ab764ada59-S77455/frameworks/1735e8a2-a1db-478c-8104-60c8b0af87dd-0196/executors/ee3b3a90-e342-4cec-983c-140eb9bea88c/runs/5c4c20ea-295b-4e87-9a70-966df9b63dd1/testOutput/test-support/jtreg_open_test_hotspot_jtreg_tier1_compiler_3/scratch/3/core.4018
#
# If you would like to submit a bug report, please visit:
# http://bugreport.java.com/bugreport/crash.jsp
#

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