[OCaml] Update api to account for FNeg and CallBr instructions

Summary:
This diff adds minimal support for the recent FNeg and CallBr
instructions to the OCaml bindings.

Reviewers: whitequark

Reviewed By: whitequark

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60680

llvm-svn: 358501