After D17993, with -fno-delete-null-pointer-checks we add the dereferenceable attribute to the this pointer.
We have observed that one internal target which worked before fails even with -fno-delete-null-pointer-checks.
Switching to dereferenceable_or_null fixes the problem.
dereferenceable currently does not always respect NullPointerIsValid and may
imply nonnull and lead to aggressive optimization. The optimization may be
related to CallBase::isReturnNonNull, Argument::hasNonNullAttr, or
Value::getPointerDereferenceableBytes. See D66664 and D66618 for some discussions.