[Attributor] Deduce "dereferenceable" attribute

Summary:
Deduce dereferenceable attribute in Attributor.

These will be added in a later patch.
* dereferenceable(_or_null)_globally (D61652)
* Deduction based on load instruction (similar to D64258)

Reviewers: jdoerfert, sstefan1

Reviewed By: jdoerfert

Subscribers: hiraditya, jfb, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64876

llvm-svn: 366788