[llvm] [BOLT][merge-fdata] Only validate field count on last line (PR #189127)

[Maksim Panchenko via llvm-commits]

https://github.com/maksfb commented:

Hi YongKang, a couple of questions: what's the effect/speedup from this PR? Can we run a light-weight validation on entries not at the end of the file? Or we rely on tools running after `merge-fdata` to catch bad entries in the middle of a file and running extra checks here is not valuable?

https://github.com/llvm/llvm-project/pull/189127