[llvm] [nfc][InstrProf]Simplify instrumentation profile reader and writer code (PR #93346)

[Mingming Liu via llvm-commits]

minglotus-6 wrote:

I'm planning to resume https://github.com/llvm/llvm-project/pull/88212 after this clean-up.

 https://github.com/llvm/llvm-project/pull/88212 will add at least 2 new header fields. This simplification can reduce code complexity there.

https://github.com/llvm/llvm-project/pull/93346