[llvm] [BOLT][AArch64] Introduce SPE mode in BasicAggregation (PR #120741)

[Paschalis Mpeis via llvm-commits]

paschalis-mpeis wrote:

Hey @yota9,

Thanks a lot for your review!

I addressed your comments except [this one](https://github.com/llvm/llvm-project/pull/120741#discussion_r1894153391) (left a comment there).
Please have another look and let me know of any further changes.

https://github.com/llvm/llvm-project/pull/120741