Detect truncating Phi uses of Phis with constant inputs

Review URL: https://codereview.chromium.org/14260013

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14525 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Turn the load field code stub into a hydrogen code stub.

Review URL: https://chromiumcodereview.appspot.com/14847004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14526 ce2b1a6d-e550-0410-aec6-3dcde31c8c00