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

[Paschalis Mpeis via llvm-commits]

paschalis-mpeis wrote:

This PR is an implementation of the (4a) approach of:
- #115333

We did some limited, quick testing and there was no clear winner between the two approaches, but the `--spe` flag is introduced in a way to accommodate both.

I believe @kaadam  had some work on (4b)? Maybe at some point we could additionally have that merged, and community can test on a wider set of apps/workloads. I believe there won't be dramatic performance changes.

Please give SPE a try along with this patch and report any feedback. To check if SPE is available on your machine, see point [(3)](https://github.com/llvm/llvm-project/issues/115333) on the issue. Let us know if more information is needed on how to enable or use SPE!

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