[llvm] [AArch64] Treat NOP as a separate instruction. (PR #170968)

[Harald van Dijk via llvm-commits]

hvdijk wrote:

Context: This is part of a larger effort to avoid instructions being unnecessarily marked as having unmodelled side effects. For LLVM, it is largely harmless, it just results in reduced optimisations. For BOLT though, it complicates binary analysis.

https://github.com/llvm/llvm-project/pull/170968