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

[Mikael Holmén via llvm-commits]

mikaelholmen wrote:

Hi @samolisov 

Did you see that a buildbot failed with this patch? I see the same if I compile with EXPENSIVE_CHECKS set and run lit tests.
The following tests start failing with this patch:
```
Failed Tests (4):
  Clang :: CodeGen/pgo-instrumentation.c
  LLVM :: Transforms/PGOProfile/critical-edge-threshold.ll
  LLVM :: Transforms/PGOProfile/declarations_only.ll
  LLVM :: Transforms/PGOProfile/global_variables_only.ll
```
They all hit the error
```
Module changed by PGOInstrumentationGen without invalidating analyses
```

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