Simplify prototype chain walk in hydrogen

Since we know that we're only encountering JSObjects, we can use the
faster GetPrototype() method to walk up the chain

BUG=none
R=verwaest@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/321543002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21718 ce2b1a6d-e550-0410-aec6-3dcde31c8c00