[llvm] [AArch64] Replace uses of AArch64::NoRegister with Register() or isValid() NFC (PR #220227)

[via llvm-commits]

janr-bay wrote:

@topperc Yes that would be appreciated

Looked into the broken test in CI. It was  just introduced before the rebase and reverted, again

https://github.com/llvm/llvm-project/pull/220227