[llvm] [BOLT] Support profile density with basic samples (PR #137644)

[Maksim Panchenko via llvm-commits]

maksfb wrote:

> > members that used to have "branch" in the name now use "sample" which means "sample or branch", which makes "sample" ambiguous.
> 
> "Sample" is supposed to cover different profile types. We disambiguate it by adding "basic" or "branch" to it. So I'd say this change generalizes stats that were only applicable to branch samples to cover basic samples as well. Do you agree?

In general, outside of context of `DataAggregator`, I agree. But in the code, we refer to non-LBR samples as just samples.

https://github.com/llvm/llvm-project/pull/137644