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

[Amir Ayupov via llvm-commits]

aaupov wrote:

> Let me share my understanding on the LBR format to see if I got this right:
Each LBR event gets a contiguous stack of taken branches. And any other branches that may lay in between them are known to be fall-throughs, which BOLT can infer. eg, if we have:

Right, with taken branch stacks, we automatically "infer" fall-throughs between entries. But if we were to collect single branch records, as I assume should be possible with SPE, we'd need to use `infer-fall-throughs` to perform inference on a CFG _after_ edge counters are attached.

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