This separates the core machinery and the heuristics involved with
inlining functions calls. So far the heuristic only respects our
%SetForceInlineFlag hint, but it will the place where general inlining
heuristics can live without impeding clarity of the core machinery.

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/1391903002

Cr-Commit-Position: refs/heads/master@{#31150}