Avoid write barriers when storing values in the root set, and use
cheaper write barriers for storing maps or tagged pointers. Also
improve the generated code for write barriers, utilizing the out
of line code mechanism that is available to TurboFan backends,
which moves the unlikely case out of the hot path.

R=jarin@chromium.org, mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/1414183006

Cr-Commit-Position: refs/heads/master@{#31914}