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

[YongKang Zhu via llvm-commits]

yozhu wrote:

Hi Maksim,

I ran some tests and saw real and sys times are almost same while user time is 3x longer. The validation is to check number of fields in a line, so it is the same check whether or not it is the last line. If `merge-fdata` succeeds, no downstream tool will validate lines in raw fdata file.

This PR was made on top of the changes in #183187, which is to catch truncated line resulted from unexpected app exit, so I was thinking checking the last line is sufficient.

On a second thought, the current scheme of checking every line offers complete coverage. Given real time doesn't diff much (at least on my local machine which is relatively powerful), we can afford running validation on all the lines.

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