[llvm] boilerplate for new callbrprepare codegen IR pass

Because this pass is to be a codegen pass, it must use the legacy pass
manager.

Link: https://discourse.llvm.org/t/rfc-syncing-asm-goto-with-outputs-with-gcc/65453/8

Reviewed By: aeubanks, void

Differential Revision: https://reviews.llvm.org/D139861