[Attributor] Remove dereferenceable_or_null when nonull is present

Summary: This patch prevents the simultaneous presence of `dereferenceable` and `dereferenceable_or_null` attribute

Reviewers: jdoerfert, sstefan1

Reviewed By: jdoerfert

Subscribers: lebedev.ri, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70789