[llvm] [RISCV] Replace uses of RISCV::NoRegister with Register(), isValid(), or operator bool. NFC (PR #161781)

[Sam Elliott via llvm-commits]

https://github.com/lenary approved this pull request.

Two quibbles, but neither of them major enough to block this.

I don't understand when you've chosen to use `Register::operator bool` and `Register::isValid`, especially when using one of those two on the result of `MCOperand::getReg`.

https://github.com/llvm/llvm-project/pull/161781