If the type of a tracked field or element value is less precise than the
advertised type of the field or element load, then we replace the load
operation with a TypeGuard that guards the advertised type.

R=jarin@chromium.org
BUG=v8:5267

Review-Url: https://codereview.chromium.org/2295643002
Cr-Commit-Position: refs/heads/master@{#39032}