[llvm] [SelectOptimize] Enable by default; add no-PGO density heuristic; fix… (PR #200691)

[Eitan Frachtenberg via llvm-commits]

eitanf wrote:

> Is there a reason that this PR cannot be split into multiple PRs based on the delineations in the description?

Can you suggest the split you'd like to see? 

https://github.com/llvm/llvm-project/pull/200691