Added LLVM metadata to generated IR to increase vectorization width

Summary: Edited `loop_proto_to_llvm` to emit metadata at the end of the generated IR. This metadata will increase the vector width when the IR is optimized.

Reviewers: morehouse, kcc

Reviewed By: morehouse

Subscribers: cfe-commits, llvm-commits

Differential Revision: https://reviews.llvm.org/D50530

llvm-svn: 339392