The problem is that tagged slot could become a double slot after migrating of an object to another map with "shifted" fields (for example as a result of generalizing immutable data property to a data field).
This CL also adds useful machinery that helps triggering incremental write barriers.

BUG=chromium:454297, chromium:465273
LOG=Y

Review URL: https://codereview.chromium.org/988363002

Cr-Commit-Position: refs/heads/master@{#27141}