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

[Paschalis Mpeis via llvm-commits]

https://github.com/paschalis-mpeis commented:

Hey Adam,

Thanks for the PBT test. Regarding this comment:
```cpp
// If the PBT feture is availabe, an SPE sample has two entries.
// These two entries form a chain of two consecutive branches.
```

With regular SPE, we only see consecutive branch pairs in code. If we try to infer any FT branches in between, we'll always get an empty set.

TMU, with the optional `FEAT_SPE_PBT`, we get an BRBE-like stack with a depth of 1?
@mate-stodulka, can you confirm that and expand with some reference?

Also, just to double-check with @aaupov, since we use BOLT's LBR format (branch stacks), fall-throughts should be inferred automatically (which PBT can benefit), so no extra flags needed, correct?

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