MIPS: [turbofan] Clean up and factor out branch generation logic.

Port aed26c05a50b2a1257ef8accc67143825f2e1f60

BUG=
R=paul.lind@imgtec.com

Review URL: https://codereview.chromium.org/742243003

Cr-Commit-Position: refs/heads/master@{#25451}