[LLVM-C][Ocaml] Add MergeFunctions and DCE pass

MergeFunctions and DCE pass are missing from OCaml/C-api. This patch
adds them.

Differential Revision: https://reviews.llvm.org/D65071

Reviewers: whitequark, hiraditya, deadalnix

Reviewed By: whitequark

Subscribers: llvm-commits

Tags: #llvm

Authored by: kren1

llvm-svn: 373170