[llvm] [BOLT][AArch64] Check Last Element Instead of Returning `nullptr` in `lookupStubFromGroup` (PR #114015)

[Paschalis Mpeis via llvm-commits]

https://github.com/paschalis-mpeis approved this pull request.

Thanks for adding the test. LGTM!

Please consider addressing the below nits.

https://github.com/llvm/llvm-project/pull/114015