[turbofan] Remove the JSGraph dependency from the ControlFlowOptimizer.

[turbofan] Remove the JSGraph dependency from the ControlFlowOptimizer.

The control flow optimization should work independent of the JSGraph. We
used the JSGraph there because it was convinient, not because it was
necessary.

R=jarin@chromium.org

Review URL: https://codereview.chromium.org/1160863003

Cr-Commit-Position: refs/heads/master@{#28674}