[llvm] [llvm-profgen] Improve sample profile density (PR #92144)

[Lei Wang via llvm-commits]

wlei-llvm wrote:

> > However, there is a problem that a high total sample profile can have a very low density, which makes the density number not as representative.
> 
> How big is the problem? Like do you see this old heuristic preventing us from getting a consistent density threshold as proxy for performance, because density is less stable?

I didn't dive very deep to the old heuristic, I just guess from the result, I saw many cases show super big number, for example and this come from normal profile. 
```
Sample PGO is estimated to optimize better with 3633.9x more samples.
```
That said, as I mentioned in the above comments, there seems a few bugs in the previous work, (another "bug" is it use the total_samples to decide the hot functions(the cutoff num is a block level count)), maybe it's caused by other issue. Anyway, it seems it needs a big overhaul.  

https://github.com/llvm/llvm-project/pull/92144