[llvm] [AArch64] Separate PNR into its own Register Class (PR #65306)

[Sander de Smalen via llvm-commits]

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+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
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sdesmalen-arm wrote:

> If you'd rather I combine them into a single file and make them near carbon copies of each other that's fine by me
That would have my preference, thanks!

https://github.com/llvm/llvm-project/pull/65306