[msan] Handle x86_avx_cmp_pd_256 and x86_avx_cmp_ps_256

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.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D134119