[llvm] [NFC][AArch64] isPureCmp is a duplicate of canAdjustCmp, so remove the duplicate (PR #183568)

[Hussam Alhassan via llvm-commits]

hussam-alhassan wrote:

Yes this duplicate function would be removed as part of #183532, but in hindsight should have been removed with #182244 so completely fine

https://github.com/llvm/llvm-project/pull/183568