[llvm] Return nullopt if Reg is undef. (PR #155893)

[Shubham Sandeep Rastogi via llvm-commits]

rastogishubham wrote:

@adrian-prantl cherry-pick of https://github.com/swiftlang/llvm-project/pull/11255 to llvm.org/main

https://github.com/llvm/llvm-project/pull/155893