Fix the compilation issue for turbofan unsupported backend which
is introduced by https://chromium.googlesource.com/v8/v8/+/aed26c05a50b2a1257ef8accc67143825f2e1f60.
( [turbofan] Clean up and factor out branch generation logic.)

BUG=

Review URL: https://codereview.chromium.org/748673002

Cr-Commit-Position: refs/heads/master@{#25452}