RFR: 8256046: Shenandoah: fatal error: DEBUG MESSAGE: unexpected null in intrinsic

[Roman Kennke]

Testing found this problem (very rare/hard to reproduce):

# Internal Error (d:/a/jdk/jdk/jdk/src/hotspot/cpu/x86/macroAssembler_x86.cpp:880), pid=6160, tid=5828
# fatal error: DEBUG MESSAGE: unexpected null in intrinsic

Testing:
 - [ ] hotspot_gc_shenandoah
 - [ ] tier1+Shenandoah
 - [ ] tier2+Shenandoah

-------------

Commit messages:
 - 8256046: Shenandoah: fatal error: DEBUG MESSAGE: unexpected null in intrinsic

Changes: https://git.openjdk.java.net/jdk/pull/1120/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1120&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256046
  Stats: 8 lines in 1 file changed: 7 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1120.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1120/head:pull/1120

PR: https://git.openjdk.java.net/jdk/pull/1120