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

[via llvm-commits]

WenleiHe 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? 

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