[turbofan] Make VisitBranch and friends to backend specific.

This is required to fix a tricky branch fusion bug on ARM, which was
caused by the interaction between the architecture-independent and
architecture-specific parts of the InstructionSelector. In the end it
wasn't worth sharing a few common lines of code for the additional
complexity, especially if we also want to properly support architectures
without any dedicated flags register (i.e. MIPS).

TEST=mjsunit,unittests
R=dcarney@chromium.org

Review URL: https://codereview.chromium.org/651783003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24546 ce2b1a6d-e550-0410-aec6-3dcde31c8c00