Allow merging of monomorphic accesses to tracked fields.

Also add stability dependency only on maps that can transition,
and delay adding the dependencies until we are actually using
them, either in a HLoadNamedField or an HCheckMaps.

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

Review URL: https://codereview.chromium.org/239923004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20796 ce2b1a6d-e550-0410-aec6-3dcde31c8c00