Homogenizes map checks for all polymorphic named field loads.
Tests show no perf regressions

BUG=

Review URL: https://codereview.chromium.org/14224009

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14259 ce2b1a6d-e550-0410-aec6-3dcde31c8c00