Removed FIXME which looks irrelevant. The error message happens only without -mattr=+avx.
E.g.
GOOD: opt llvm/test/Instrumentation/MemorySanitizer/avx-intrinsics-x86.ll -passes=msan -o - | llc -O3 -o /dev/null -mattr=+avx
BAD: opt llvm/test/Instrumentation/MemorySanitizer/avx-intrinsics-x86.ll -passes=msan -o - | llc -O3 -o /dev/null
So nothing to fix here.
Have you been able to determine whether this has been addressed?