Rename HBranch (the instruction that coerces an arbitrary HValue to
control flow) to HTest to free up the term Branch to refer to any
control instruction with two successors.

Change the virtual FirstSuccessor and SecondSuccessor functions on
control instructions to a pair of data members.

Review URL: http://codereview.chromium.org/6366002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6417 ce2b1a6d-e550-0410-aec6-3dcde31c8c00