Inline refactoring: revise candidacy determination

Inline refactoring: revise candidacy determination

Rework the code to determine the key aspects of candidacy up front
rather than figuring it out eventually. In particular this ensures
that the two stages of evaulation for inlines both start at the same
point for candidates.

This will help streamline subsequent work to move the state machine
into the policy, since the state machine is only needed for the
case where the candidate is a discretionary inline, and the policy
now tracks this directly.

For the `LegacyPolicy`, if an inline is both smaller than the always
inline size and marked force inline, attribute its inlining to its
small size.

Remove bulk of the of the "evaluation in progress" candidate
observations since they don't add much value. As a result `noteCandidate`
is not needed as an external API, and can be repurposed internally as
`setCandidate`.

Change how the prejit root case is tracked to make it explicit
from the context rather than a callback to the compiler.

Commit migrated from https://github.com/dotnet/coreclr/commit/96783862204ea1dd91e55ef4c44179535ff68514