[HotColdSplit] Mark inherently cold functions as such

[HotColdSplit] Mark inherently cold functions as such

If an inherently cold function is found, mark it as cold. For now this
means applying the `cold` and `minsize` attributes.

As a drive-by, revisit and clean up the criteria for considering a
function for splitting. Add tests.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@351623 91177308-0d34-0410-b5e6-96231b3b80d8