[llvm] [PGO] Preserve analysis results when nothing was instrumented (PR #93421)

[Pavel Samolysov via llvm-commits]

samolisov wrote:

@teresajohnson @aeubanks Could you have a look at the PR again? I believe, all the comments and suggestions were addressed. Even though the next parts of this "stacked" PR won't pass the review, this part may reduce the compilation time a little bit, at least.

https://github.com/llvm/llvm-project/pull/93421