We (mis)used Type::Class to track stable field maps in the past. But
that always more or less unsupport and wrong for various reasons, mostly
because the class types do not really present static information and
thus it is possible to violate fundamental assumptions of the type
system (i.e. intersecting class types and other types produces
"interesting" results).

Now it is possible to finally nuke the class types completely and thus
simplify (and ideally correctify) the type system further.

Note to performance sheriff: We do expect to see some performance
regressions from this change. This is because we do not yet have a sane
replacement mechanism to track known field maps and utilize them during
LoadElimination. This will be accomplished in a follow up CL.

BUG=v8:5270,v8:5267
R=jarin@chromium.org

Review-Url: https://codereview.chromium.org/2293343002
Cr-Commit-Position: refs/heads/master@{#39031}