Shenandoah: assert(mem == __null) failed: only one safepoint

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

java/net/httpclient/whitebox/Http1HeaderParserTestDriver.java fails intermittently with Shenandoah:

# A fatal error has been detected by the Java Runtime Environment:
#
# Internal Error (/home/shade/trunks/jdk-jdk/src/hotspot/share/gc/shenandoah/c2/shenandoahSupport.cpp:2312), pid=30616, tid=30642
# assert(mem == __null) failed: only one safepoint

Steps to reproduce:

$ for I in `seq 1 20`; do CONF=linux-x86_64-server-fastdebug make images run-test TEST=java/net/httpclient/whitebox/Http1HeaderParserTestDriver.java TEST_VM_OPTS="-XX:+UnlockExperimentalVMOptions -XX:+UseShenandoahGC"; done

Bisection says it started happening after JDK-8234049, so probably something had changed in the code shape there.