[llvm] [MC] Make MCRegisterInfo::getLLVMRegNum return std::optional<MCRegister>. NFC (PR #107776)

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

@topperc I'm assuming the plan here is to eventually remove the implicit integer conversion from `MCRegister`?

https://github.com/llvm/llvm-project/pull/107776