[BasicAA] Use dereferenceability to reason about aliasing

Summary:
We already use the fact that an object with known size X does not alias
another objection of size Y > X before. With this commit, we use
dereferenceability information to determine a lower bound for Y and not
only rely on the user provided query size.

The result for @global_and_deref_arg_2() and @local_and_deref_ret_2()
in test/Analysis/BasicAA/dereferenceable.ll improved with this patch.

Reviewers: asbirlea, chandlerc, hfinkel, sanjoy

Subscribers: hiraditya, bollu, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66157

llvm-svn: 369786