[llvm] [BOLT] Adding a unittest that covers Arm SPE PBT aggregation (PR #160095)

[via llvm-commits]

mate-stodulka wrote:

Hi, 
The architecture in the Arm Architecture Reference Manual [version L.b](https://developer.arm.com/documentation/ddi0487/latest/) defines the Previous Branch Target as follows:

> FEAT_SPE_PBT provides support for generating a packet that provides the target address for the previous taken branch.

Section D17.6.3.1 provides more detail, but the relevant part is that PBT records only _taken_ branches. That is the same stipulation as the BRBE has, so the PBT should behave as the target field of a single branch record. Which means fall-throughts  between PBT and the current SPE sample's source are possible. Hope that helps clarify things!

https://github.com/llvm/llvm-project/pull/160095