[llvm][AArch64] Add missing default cases (#89930)

Addresses issue #89709.

Functions getAUTOpcodeForKey() and getPACOpcodeForKey() contain switch
statements without default cases. Resolved by adding a call to
llvm_unreachable() at the end of each function.

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Signed-off-by: Troy-Butler <squintik@outlook.com>
Co-authored-by: Troy-Butler <squintik@outlook.com>