[llvm] [ARM] Fix failure to register-allocate CMP_SWAP_64 pseudo-inst (PR #104039)

[Oliver Stannard via llvm-commits]

ostannard wrote:

@arsenm The AMDGPU backend uses AllocationPriority so this causes a lot of test changes, because that was previously ignored for fast-regalloc. Do you think this change is OK for AMDGPU, or should I make it specific to ARM?

https://github.com/llvm/llvm-project/pull/104039