[llvm] [BOLT][merge-fdata] Fix basic sample profile aggregation without LBR info (PR #118481)

[Paschalis Mpeis via llvm-commits]

paschalis-mpeis wrote:

Just adding that there is a great overlap with:
- #112328

A difference (ignoring the formatting changes) is that `split-file` was used, requiring no specific `.fdata` files.
If this gets fully accepted & merged, I see the other PR to be closed?

https://github.com/llvm/llvm-project/pull/118481