We could try to treat an HPhi as an HInstruction because the code did
not properly handle the case of a phi in a block with itself as one of
the predecessors.

BUG=v8:1134

Review URL: http://codereview.chromium.org/6471020

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6721 ce2b1a6d-e550-0410-aec6-3dcde31c8c00