[turbofan] Make VisitBranch and friends to backend specific.

[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

Committed: https://code.google.com/p/v8/source/detail?r=24546

[dcarney, 2014-10-13 07:39:21]

lgtm

a unit test would be nice...

[Benedikt Meurer, 2014-10-13 08:11:47]

> a unit test would be nice...

Hm, this is really weird. Depends on so many conditions, probably the reason why
zlib is currently the only repro. Cannot seem to come up with anything
maintainable currently...

[Benedikt Meurer, 2014-10-13 08:15:15]

Committed patchset #2 (id:110001) manually as 24546 (presubmit successful).