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

[Pavel Samolysov via llvm-commits]

samolisov wrote:

I'm going to rebase my changes onto updated main because an important part of the work: unit tests and the baseline for LIT tests are already there. I know, there is some troubles with rebased commits in GitHub UI, but I see no alternative. Maybe merging main into my branch but I afraid of a lot of intermediate commits will be displayed as changes in the PR and make the review impossible.

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