I found this peculiar comment in EarlyCSE:
1c78d8d9d7/llvm/lib/Transforms/Scalar/EarlyCSE.cpp (L1620-L1624)
Looking back over history, this seems to be referring to the
aarch64.neon.stN intrinsics, which are indeed not marked writeonly
(though the ldN intrinsics are readonly).
Possibly I'm missing something special about these intrinsics, but I
think it is safe to mark them as writeonly.