Track field types.

This is an initial step towards tracking the exact types instead of just the representations of fields. It adds support to track up to one map of heap object field values, eliminating various map checks on values loaded from such fields, at the cost of making stores to such fields slightly more expensive.

TEST=mjsunit/field-type-tracking
R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/167303005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20701 ce2b1a6d-e550-0410-aec6-3dcde31c8c00