MIPS: [turbofan] Make VisitBranch and friends to backend specific.

MIPS: [turbofan] Make VisitBranch and friends to backend specific.

Port r24546 (c3443cd)

Original commit message:
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
BUG=

[paul.l..., 2014-10-13 23:27:58]

paul.lind@imgtec.com changed reviewers:
+ akos.palfi@imgtec.com, bmeurer@chromium.org, danno@chromium.org,
dusan.milosavljevic@imgtec.com, paul.lind@imgtec.com

[paul.l..., 2014-10-13 23:27:58]

This is a preliminary port of r24546, which should unbreak the build for mips.
I'm fixing 2 small issues at check in, and reverting the change to
instruction-selector.cc, which fixes non-TF backends. (I don't have permission
to approve that, and will submit under a separate CL).

With these caveats, LGTM.

[Benedikt Meurer, 2014-10-14 11:16:42]

lgtm

[paul.l..., 2014-10-14 17:28:04]

Committed as 24571. (on 13-September)