[CodeGen] -fno-delete-null-pointer-checks: change dereferenceable to dereferenceable_or_null
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.
Reviewed By: bkramer, rsmith
Differential Revision: https://reviews.llvm.org/D92297