[Attributor] Manifest alignment in load and store instructions

Summary:
We can now manifest alignment information in load/store instructions if
the pointer is known to have a better alignment.

Reviewers: uenoku, sstefan1, lebedev.ri

Subscribers: hiraditya, bollu, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66567

llvm-svn: 369804