[llvm] [LFI][AArch64] Add AArch64 LFI rewrites for system instructions (PR #186896)

[Zachary Yedidia via llvm-commits]

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zyedidia wrote:

I think doing this cleanly would require changing the `error` function to accept a `const Twine &` instead of a `const char[]`. I think it's reasonable to leave this for a future patch for now, but I can make the change here if desired.

Additionally, in the future we could store "virtual" versions of the reserved registers in the context area and automatically rewrite accesses, which would allow processing of hand-written assembly code that uses the reserved registers (at some performance cost for those instructions).

https://github.com/llvm/llvm-project/pull/186896